Would the Buddha have considered engaging in anal sex as breaking the 1st and/or 5th precept?

If you do any sexual act with someone under guardianship, protection or care, then it breaks the 3rd precept, regardless of the nature of the act. The party providing guardianship, protection or care can be oneself, partner, family, government, religious institution, etc.

The 1st precept is about killing and the 5th precept is taking intoxicants.

In VinayaPitaka Pathamaparajika, Pathamaparajika precept will be perfectly completely broken when bhikkhu's penis immediately touching 3 holes, v***na, anal, and mouth.

Exposition part 2

Voluntary sexual intercourse

If a monk has the intention of association and he lets his penis enter a human female through the anus … through the v***na … through the mouth, he commits an offense entailing expulsion. If a monk has the intention of association and he lets his penis enter a non-human female … a female animal … a human hermaphrodite … a non-human hermaphrodite … an animal hermaphrodite through the anus … through the v***na … through the mouth, he commits an offense entailing expulsion. If a monk has the intention of association and he lets his penis enter a sexually nonconformist human … a sexually nonconformist spirit … a sexually nonconformist animal … a human male … a male spirit … a male animal through the anus … through the mouth, he commits an offense entailing expulsion.

It doesn't break the first precept because sperm is not a sentient being.

It doesn't break the fifth precept because anal sex is not an intoxicating substance like alcohol or recreational drugs.

It may break the third precept, but that depends on WHO the sex partner is, rather than WHAT the sex act is. You can read this answer for details.

Obviously, anal sex performed by a husband on a wife would not break the literal teaching found in the 3rd precept. However, its often voyeuristic nature tends to place it within the sphere of mere lust, which is unwholesome, unless the wife genuinely rather than begrudgingly consents to the act.

In certain societies, anal sex was a common way for men to have sex with their wives and avoid pregnancy. However, many wives resented it. If a wife resented this, it would be an unwholesome act, not based in real love. But if the wife enjoyed it & was happy to do it, I suppose it would be OK (ignoring the fact the anus being something not really designed for sexual penetration).

Similarly, if committed h*m*sexual partners sincerely wish to practise anal sex, it seems OK, I suppose (i.e., ignoring whatever risks are involved). But if anal sex is part of some domination or aggression motivation, it would not be OK.

Sex exists in nature for reproduction. However, it is acknowledged in religion that sexual desire is also something "oppressive"; that often needs to be "managed". When questions about sexual morality arise from the perspective of "sexual desire management", it becomes more complex & less straightforward. But when sex is for reproduction, sexual morals is easier to define.

Naturally, ordinary male & female relationships have children as a very strong factor that naturally binds the relationship. When people, be they h*m*sexual or heterosexual, engage in childless relationships, this lacks the natural children factor that helps define ethics and also greatly helps maintain the relationship.

For example, when a wise man sees marriage & children are the optimal life for a woman, a wise man can easily generate compassion towards women and even lose sexual desire towards women (and thus easily become a monk or enter samadhi). In the Buddhist suttas, wise men left the household family villlage life because they could see the business of reproduction & family life was a burden & bondage. For h*m*sexuals, however, such a perspective is lacking.