What are the historical reasons for religious and political negative attitudes towards homosexuality?

Speaking for Europe: People in the past used to be very religious, almost all of them. And they (tried) to follow very strictly the Bible, what to do and what not to do. Since the Bible condemned h*m*sexuality, it was a no go for all countries.

Homosexual relationships do not lend themselves well to marriage, as discussed below. Therefore, they took place outside of marriage, and would be regarded as a form of "adultery," or at least "fornication."

The REAL sin of a h*m*sexual relationship is that it cannot produce children BY DEFINITION. That' why until recent times, there was no such thing as "gay marriage." And even if gays can marry (as today), there is no MATRIMONY (basically, "mother-making") involved.

Traditional societies were "all about" producing children, since most of them didn't live to adulthood. Any practice that stood in the way of producing children (h*m*sexuality, birth control, masturbation), was therefore frowned upon until modern times.

This is a bit of a tricky question. Before speaking of historical views of h*m*sexuality, it is very important to understand what is being spoken about. Prior to the 20th century, there really was no concept of h*m*sexuality as we understand it today. In the late 19th century, the concept of h*m*sexuality as an actual orientation was first proposed and referred to as 'sexual inversion.' (And that was in a book meant for mostly academic audiences, so it was decades before this became a widespread social idea.)

Prior to that point, there simply was no concept of h*m*sexuality as we understand it. There were many groups which opposed h*m*sexual ACTS, but these were mostly viewed as crimes like theft, murder, etc. While there were certainly people who preferred to commit these crimes more than others, just like there are career thieves and petty thieves, the idea that those people would necessarily oppose heterosexual relationships was unknown. Likewise, the idea that a person was unlikely to commit h*m*sexual acts simply because they are involved in a heterosexual relationship was unknown.

Today we believe that h*m*sexuality sets a person apart as a different KIND of person. It's not simply an act which anyone is liable to commit and which does not necessarily have wide-ranging implications about the person who commits it. It is important to keep this in mind when reading about historical attitudes toward h*m*sexuality. Committing h*m*sexual acts was seen as a moral failing, but not a systemic one.

So why was it a moral failing at all? It mostly comes down to incorrect understanding of the purpose of human sexuality. In ancient Greece, for example, sex was seen as a power display. Those who penetrated were more powerful than those who were penetrated. Most of our modern ideas about sexuality are inherited from ideas that first gained widespread acceptance during the Industrial Revolution.

The Industrial Revolution, and the fact it created a situation where adolescents were mostly no longer able to get a job which paid enough to support a family, led to widespread acceptance of the idea that bodily pleasure was destructive. In religious circles it was seen as impious, in secular circles it was seen as unhealthy. Basically any form of pleasure, including sexual pleasure, was seen as a potentially lethal weakness. People who were 'slaves to their passion' were seen as weak and corrupt. It all came down to productivity. Homosexual acts produced pleasure, but could not produce children. Pleasure was seen as a dangerous temptation, and failing to produce anything seen as a moral failure.

It is important to note that even during the Industrial Revolution, the time during which religious ideas opposing sex really took center stage (they existed previously but were like Catholic views on contraception today... simply ignored in most cases. Families slept, and screwed, in common rooms. Children grew up exposed to sex from birth and the lower classes at least saw no reason to prohibit child sex play so it was universal and that led to a whole life filled with sex. What else did the lower class have to do for fun?), their opposition was quite different from the opposition we see today. They had, as did every culture except modern western cultures, a category of human relationship which we have somehow managed to completely eliminate - platonic romance.

Platonic romances were romantic, but supposedly nonsexual, relationships between same-sex persons. Even married men and women were likely to have same-sex friends which they viewed romantically. They would exchange gifts, write love poems and songs for one another, pledge undying love, etc. When modern people read of these relationships they project modern ideas of h*m*sexuality onto them which is really inappropriate and ignorant. They simply did not tie sex and love together the way we do. Sex was seen first as an entertainment, then as a lamentable necessity of the human animal.

Though not related to h*m*sexuality, I feel it important to mention the view of sexuality prior to this. Development of agriculture had even more impact on how people viewed sexuality, and we retain some of those beliefs today. It was the birth of sex being seen as something which should be controlled. Unlike tribal situations, where children were raised amongst the entire tribe, once people settled down and started agriculture it became very expensive for a man to raise a child which was not his own. This motivated men to control womens sexuality. This started the idea that a mans spouse and children were property of the man. 'Deflowering' a mans daughter was seen as a PROPERTY crime. We still retain vestiges of this. When someone speaks of "saving myself", and otherwise uses economic language when speaking of sexuality, this is where it came from. They are "saving" their value for a dowry for their father.

Sources: 'Masturbation: The History of a Great Terror', Michel Foucault's 'The History of Human Sexuality', 'Good Sex Illustrated', 'The Sociology of Sex' course from The Teaching Company, ('Sex At Dawn', 'Sex At Dusk' always read together)

Expanding Tom Au's answer:

"Homosexuality" as we know it is a product of 19th century medical science which ties together a sequence of sexual, emotional, family and social conducts and gives them a new name. We should remember that the conducts we think of as a behaviour "h*m*sexuality" were not necessarily perceived of as a unitary conduct in past societies. That element of theory out of the way:

People who did not marry, and had property to transmit through marriage, would be disciplined by their family or society. This strongly incentivised marriage, and in many agricultural communities marriage was practiced as male to female only. The reasons for male to female only marriage are a combination of reproductive and property based. While he is incredibly out of date in this regard, Engels provides an engaging description of human marriage in Family, Private Property and the State. It is worth it to the extent that Engels notices that the family and marriage are conditioned by other social phenomena.

Marriage was an economic institution (oikos: household). It brought male, female or child labour into the household or established new households. There was, therefore, regulation over marriage and strong economic incentives towards marriage.

Sexual and emotional conduct outside of marriage was often perceived to threaten the system of marriage. In part this is because sexual conduct outside of marriage leads to children, and children can be used to form households. Emotional conduct outside of marriage has been perceived to lead to sexual conduct (even courtly romance notes this), and this can lead to the getting of children. And the getting of children can lead to non-ideal marriages from a property standpoint, or to other problems.

However, sometimes marriage can produce what we now analyse as "h*m*social" situations. Monasteries. Warrior cults. Female household networks. Or the structure of the family (patriarchal absolutism say) gave certain people the power to configure permissible sexual expressions outside of marriage.

Also, it is worth noting that officially sanctioned violations of the cultural traditions above existed. Elites often displayed power by violating cultural norms that applied to others.

These complex configurations of gender and sexual expression, when expressed as a continuing culture, can be linked to property. But specific moments have more complex causes than simple one to one links.

The historical position of conducts like forming strong couple bonds, households, marriages, reproductive, economic or sexual relationships are bound up in property, family and gender. Some accounts of post-19th century h*m*sexuality emphasise this in the connection between the "nuclear" family and imperialist capitalism.