Why was there so little information about sex in Eastern Block countries?

Was it "taboo"?

This is just not correct on a lot of levels.

First of all, the "Eastern bloc" was not a so much like a real 'bloc'. Differences existed for each and every country. Then over time, things changed, 'naturally'. In fact quite 'naturally'. For the German Democratic Republic, Hungary, things were probably a lot more relaxed than in Poland. Then we see a few processes of change at work over a timespan of 70 years for the Sovit Union and 40 years for the rest of this 'bloc'.

The TV aspect might be quite misleading here. On mainstream television a "wardrobe malfunction" around Mrs Jacksons torso for a few seconds during the superbowl is still grounds for a prudish scandal on American TV and ongoing censorship efforts. I guess we don't talk HBO level nudity where drama seems sometimes arouse solely out of naked breasts shown for not much of an apparent reason. Such a thing was also a scandal in 1950s West German TV, showing a bare breasted woman in a blurred background of a painting scene could still rally the masses for public outcries. And the 1950s were equally repressive in East Germany concerning sex. The just mentioned movie was shown to outrage in the West, while in the East it was just completely unthinkable at the time.

But the East German TV produced for example the police procedural Polizeiruf 110. Showing a female lead 'detective' (Leutnant Arndt) from the start in 1971 onwards. Later they seem to have been wanting to make an HBO-like point and frequently showed naked people, female and male, for scenes connected to eroticism as well as just en passant. (There are collections of these scenes circulating, as they fill DVDs. Season 08 No 05 (55) from 1978 even shows the female lead naked in an extended shower scene.) This was mainstream television, prime time.

In 1978 the cinematic movie Sieben Sommersprossen (recommended viewing at 12 years up) dramatised teenage love at 14, including pill contraception, in state organised summer camps. Quasi official verdict in the movie, uttered from camp organisers after a small moral outrage over the facts: "the supervisor Benedict is of the opinion that one should not overvalue these 'immoral' things."

Some tensions and the usual shenanigans ensue, then they make peace and a Shakespeare play and afterwards all are happy socialist campers. The sex angle was made a big deal of, but then this issue is resolved as 'private business', 'most probably no reason to interfere with nature'. GDR critique of the movie at the time:

The fact that this so finely conceived story, with its ravishing dialogues, turned out to be a wonderful film is mainly due to the staging by Hermann Zschoches, but also to the camera of Günter Jaeuthes and the music of Gunther Erdmann. […] You don't necessarily have to be young or have freckles to like the film, which treats vital problems of young people so honestly, courageously and without false shame and with infectious fun.
_{– Renate Holland-Moritz: Eulenspiegel 42/1978}

"I've heard adults say, looking at the film pictures in the showcases of the 'International' [movie theatre], that they would forbid their children to see the film, where is that possible, fourteen-year-olds naked and they touch themselves. The film […] deliberately breaks taboos; it shows us: fourteen-year-olds have erotic feelings, even if they are much more hesitant and tender than many fifty-year-olds might imagine. It appeals to all those involved in education not to impose discipline and order at the expense of creativity and sensitivity. …] Seven Freckles is the most debatable DEFA film of recent times, perhaps even the most beautiful.
_{– Jutta Voigt: Sonntag 44/1978}

From the early 1960s onward, private sex activity was almost endorsed. Bathing in the nude was tolerated at rivers, lakes, and Baltic beaches (FKK). Erotic art magazines became available, albeit in low circulation, but given the censorship this was almost like 'state endorsed' therefore. "Das Magazin" (examples and analysis, an ongoing research project: "Socialism is Fun"), "Akt und Kunst", even the army newspaper "Armeerundschau" later supplied recruits with highly sought after pinup pictures, tame as those were.

The contents of "Akt und Kunst" however were a bit more explicit than the covers:

^{(search for more on the net)}

Concerning sex education, the Easterners were also far ahead. Years before the West German 'Bravo' introduced "Doktor Sommer", the GDR had "Professor Borrmann". Unlike the western copy, Borrmann wasn't a made up name for a team of journalists answering questions teenagers had concerning sexuality. He had his dissertation titled:

Sexual instruction of children and young people with special attention to the participation of the teacher of the general polytechnic secondary school in the German Democratic Republic. (1961)

His contributions were published in Neues Leben targeted at teens and a comparable outlet was permitted in the more politicised junge Welt. Neues Leben had full page nude acts at least from 1965 onwards.

Most famously, the sex education books for children and teenagers, of which the 1976 edition Heinrich Brückner "Denkst Du schon an Liebe" (Do you start to think about love?) is the best known example. It was designated as "from 12 years and up".

Pornography (explicit link), or what the censors thought constitutes 'that', wasn't allowed, as was prostitution looked down upon, not allowed, but often practiced, like in Berlin and Leipzig, the latter a place with plenty of obviously tolerated offerings during high season around the trade fair.

Sex education was introduced against public pressure in schools country wide in West Germany in 1969. The GDR made this a fact in 1959. Even earlier the communists included most of this topic in the biology curriculum. In 1947.

Until 1955, Anton Makarenko, the Soviet authority on education, provided a highly normative model for how, and how not to educate young people about sex. In his opinion: ‘If the child is educated in relation to honesty, enthusiasm for work, sincerity, straightforwardness, cleanliness, love of truth, respect for others, love of the homeland and devotion to the ideas of the socialist October revolution then we will also educate him or her with respect to sexuality.’ The primary concern was ensuring that parents produced reliable ‘human products’. […]

In the Spring of 1956, senior SED politicians appeared to endorse Junge Welt’s more liberal approach to sex and sexuality. Hilde Benjamin, the Minister for Justice, better known for orchestrating show trials, hosted a forum for exchanging ‘girl talk’ about sex, the latest fashions and shopping. Benjamin argued that it was essential for all young women (and young men) to receive information about sex and contraception.
_{— Mark Fenemore: "The Growing Pains of Sex Education in the German Democratic Republic (GDR), 1945–69", in: Lutz D. H. Sauerteig & Roger Davidson (eds): "Shaping Sexual Knowledge. A Cultural History of Sex Education in Twentieth Century Europe", Routledge: Abingdon, New York, 2009.}

A remark on the Wikipedia page for the author of Sztuka kochania, Michalina Wisłocka: it was definitively not "the first guide to sexual life in Communist countries." In 1969 Siegfried Schnabl for example published "Mann und Frau intim", with ample photographic 'scenes to learn from:'

In light of that, I'd suggest to rephrase the question:

Why do many people get this completely distorted impression about gender, sexuality and erotic arts in communist countries?

It is said that the less p**nography is available, the more the people do it for real. In East Germany, they surely did. Twice as much and twice as good compared to the West. Kristen R. Ghodsee: "Why Women Had Better Sex Under Socialism", New York Times, Aug. 12, 2017.

Considering h*m*sexuality, the GDR passed a law that practically allowed 'it' for and among adults, only sanctioning it if a minor would be involved. It wasn't really punished since the late 1950s. That new GDR law came into effect in 1968. In 1988 even that passage about adults/minors was dropped completely. Compared to the West, Paragraph 175 was kept from 1872 in mostly the form that the Nazis modified it into in 1935 until 1994! In the last year of its existence, 1994, there were still 44 people convicted for h*m*sexual acts.

… the 1960s and 1970s saw h*m*sexuality decriminalized elsewhere in the communist bloc (in Czechoslovakia and Hungary in 1962, in the GDR and Bulgaria in 1968 and in Yugoslavia in 1977)…
_{— Richard C. M. Mole: "Introduction to “Soviet and Post-Soviet Sexualities”", Slavic Review, Volume 77, Issue 1 Spring 2018 , pp. 1–5.}

And in Poland things were also not as dry as it may seem to many:

One of many little-known facts about Poland is that it was the first country in the world to have an officially licensed sexologist: Kazimierz Imieliński. Back in 1963, there was nobody in Poland quite like him. He was the first leader of the so-called Polish school of sexology – active in the 1960s, ’70s and ’80s, they created original scientific concepts for people’s sex lives and educated thousands of patients and millions of readers in a country where ‘those things’ were rarely mentioned.
_{— Natalia Mętrak: "Poland’s Masters of Sex: Therapy, Communist Censorship & The Polish Kamasutra", #heritage, culture.pl, Jun 22 2016.}

Although the achievments on that front under communism are under conservative attack now: Agata Pyzik: "Poland is having a sexual revolution in reverse "The backlash sees sex education and contraception being restricted – and gay people compared to paedophiles", The Guardian, Tue 11 Feb 2014.

Was there anything especially Marxist in this phenomenon?

For now, the parameters of this conflict can be summarized by a simple question: Was 1917 a license to experiment and defy received conventions of normative sexual behavior, or did it impose on youth added responsibilities so as to preserve their "energy" for the revolution? Perhaps no single issue captivated students' and young workers' attention as did sex after the revolution, and it was not significantly hidden from view. Commentary on sexuality found expression in the most diverse media: party platforms, sociological studies, published surveys, health brochures, literary journals, newspapers, and special handbooks. Yet the most heated and arguably far-reaching domain of debate was the fiction that employed "the question" as its motivating center. Sex, as always, sells, …
_{— Greg Carleton: "Writing-Reading the Sexual Revolution in the Early Soviet Union", Journal of the History of Sexuality, Vol. 8, No. 2 (Oct., 1997), pp. 229–255.}

That the Soviet experience was much different than in other socialist countries is not really reason to think of it as completely taboo:

The old-fashioned propriety and drabness of life in the Soviet Union suggested to visitors and observers in the post-war era that sexuality was deeply hidden, and that there was little question of a Western style ‘sexual revolution’ taking hold. The ‘socialist’ USSR lacked the capitalist West’s commercial culture that used sex to promote consumption. Soviet media were tightly controlled by a very prudish censorship. The regime forbade private, non-governmental organisation, so feminists and sex radicals, extremely rare in underground intellectual life in any case, could not agitate publicly for change. Yet social and economic change transformed sexual behaviour, and citizens challenged the regime’s sexual authoritarianism by direct and indirect means. There was a sexual revolution in the Soviet Union during the 1960s and 1970s, and it was marked by significant differences to the simultaneous revolutions in the West.

Scholars have several periodisations, and more than one location, for the ‘sexual revolution’ associated with Soviet modernity. Most historians agree that a ‘first’ sexual revolution accompanied the Bolshevik Revolution with its legal and behavioural changes of the 1920s. Yet they are divided about whether a second ‘sexual revolution’ occurred in the ‘liberal 1960s’. For some, like Igor Kon, this 1960s revolution is a social fact, traceable in objective sociological knowledge about chang- ing behaviour and attitudes. For others, a revolution without discourse is incomplete. Consider Finnish sociologist Rotkirch’s view:

In Soviet Russia, the sexual revolution ... can be said to have happened the other way round [to Finland’s]: sexual practice changed much before public ideology. In the late 1970s, many people were already living as though the sexual revolution had happened. But its spoken articulation, both private and public, only began a decade later, right at the end of the Soviet era. Indeed, in Russia today, the new public ideology is only now in the making.

From the evidence presented here (including Rotkirch’s own interviews), it is difficult to agree with the conclusion that ‘spoken articulation’ of sex whether public or private, only began after 1985 when glasnost freed the Soviet press. In the late-Soviet era, sex was articulated, publicly and privately, and the social sciences contributed to that articulation.
_{— Dan Healey: "The Sexual Revolution in the USSR: Dynamics Beneath the Ice", in: Gert Hekma & Alain Giami (eds): "Sexual Revolutions", Genders and Sexualities in History, Palgrave Macmillan: Basingstoke, New York, 2014. p236–248.}

What constitutes a 'taboo' is a societal phenomenon, one that is not necessarily ordered from top down. While biopolitics are always on the agenda, even in what's now 'the West', in the Soviet Union sexual mores were as emergent from the down up as tried to enforced by law and command. And how it is portrayed for the Bolshevik period or the later Soviet Union in the West is often highly tainted by ideological prejudice.

Specifically allegations like "free love lead to mass rape" must be taken with a very unhealthy dose of salt. It is a huge difference between actual crimes occurring, for whatever reason, if they indeed are committed, and a veritable venereal moral panic spreading and preoccupying minds in a society that just granted women much more rights over their own bodies than before. Or did 'free love' in San Francisco' summer of love lead to mass rapes? Those who do assert that that had happened are quite special.

Kon surveys Soviet policies from the revolution to glasnost and blames the government for failing to acknowledge sexuality as vital to human life. In his stinging indictment, official silence nurtured ignorance, which led to tragedy: rampant sexism, sexual abuse, rape, and abortion used as a primary form of birth control. When authorities did open their mouths, Kon is no less forgiving: “Bolshevik philosophy on gender and sexuality was as primitive as that of a cavemans club.” The details he provides establish a nightmare of failed policies that are distinguishable from each other only by the degree of malice and mistake. Indeed, the resulting disdain and ridicule only confirm the traditional view of “red love” that has prevailed since the be­ginning of the Cold War.

By contrast, Eric Naiman’s "Sex in Public: The Incarnation of Early Soviet Ideology" (1997) has put to rest the perception that sex was a taboo subject for Soviet culture. Following the lead of Michel Foucault’s History of Sexuality, this work makes a fundamental contribution to the field not only in its theo­ retical approach but also in the material accessed. Drawing on little-known medical, legal, literary, and journalistic sources, he astutely observes that in 1920s Russia “talk about sex became a metaphor — and symptom — for thoughts about something else: politics and ideology.”
_{— Gregory Carleton: "Sexual Revolution in Bolshevik Russia", University of Pittsburgh Press, Pittsburgh, 2010.}

— Kyle Frackman & Faye Stewart: "Gender and Sexuality in East German Film: Intimacy and Alienation", Boydell & Brewer, 2018.
— Josie McLellan: "Love in the Time of Communism: Intimacy and Sexuality in the GDR", Cambridge University Press: Cambridge, New York, 2011. (example, p 33)
— Eric Naiman: "Sex in Public: The Incarnation of Early Soviet Ideology", Princeton University Press, 1997.
— John Stanley: "Sex and Solidarity, 1980-1990", Canadian Slavonic Papers / Revue Canadienne des Slavistes, Vol. 52, No. 1/2 (March-June 2010), pp. 131-151.
— Kurt Starke: "Sexuelle Verwahrlosung in der DDR?", in: Michael Schetsche & Renate-Berenike Schmidt (eds): "Sexuelle Verwahrlosung. Empirische Befunde – Gesellschaftliche Diskurse – Sozialethische Reflexionen", VS: Wiesbaden, 2010. Empirical data in contrast to moral discourse.
— Lukasz Szulc: "Transnational Homosexuals in Communist Poland: Cross-Border Flows in Gay and Lesbian Magazines", Global Queer Politics, Springer, 2018. (gBooks, p169)
— Video documentary on Youtube, from French/German mainstream television : DDR Erotik - Zwischen BlümchenSex und KnetFiguren - Pornografie in der DDR