What events or social sentiments lead to the oppression of homosexuals?

Thank you for this interesting question! However it is somewhat challenging to answer, since there isn't a definitive answer. I'm warning you, this will be a long one. Also take into account that my personal point of interest is on European and recently increasingly local queer history, so this affects my point of view. I will be speaking about Europe and Western society in this answer. My specialty in particular is in the German-speaking part of Europe, so many of my examples and perspectives are influenced by that and I am mostly familiar with German language discourses on queerness.

Second of all I'm sorry that I can't really say too much about the Bible and Christian attitudes on homosexuality. However it seems to me that liberal Christians think that the parts of the Bible that condemn homosexuality have been mistranslated, altered and interpreted differently during different times in history. As you probably are aware, the Bible was written over the course of several centuries by several different people. So there are some layers there. I did informally ask a friend of mine who has studied history of the Near East and she told me that homosexuality was not necessarily always condemned in that region, so it is likely that those parts have been added to the Bible or deliberately misconstrued later. The Bible can still be viewed as one very important cause for homophobia.

Short answer to the question on the header would be: I don't know for certain. Societal attitudes towards same-sex relationships specifically have changed radically over the course of centuries, starting from whether people have even recognized that same-sex relationships exist and ending to mass imprisonment of mostly gay men in the 20th century. As to whether there were specific events that influenced this, it's not likely that any event per se can be pointed out as a root cause for this issue. As to what social sentiments led to rejection of homosexuality, I mostly blame rigid gender roles and 19th century medical science. I'll explain this further later on in my answer.

This is where I need to get into historical theory for a moment. First we need to look at how historical sensibilities around queerness generally look like. By that I mean how people imagine queer people lived and how queer people were received in the past. In Western society the idea is generally that queer people had to hide themselves away and live closeted, miserable lives in fear of being condemned, imprisoned or killed. Some people even might think that queer people didn't exist at all. Common historical imagery concerning queer people seems to be gay men cruising on the outskirts of parks, looking for casual sex in public toilets. Or a trans woman painting her face in utmost secrecy, never daring to go out in the clothes she wants to go out in. They are very true stories, and painful ones at that, but also stories that we first think of when we hear the words "queer history".

However, the truth is always much more complicated, so this idea that it was always so very difficult to be queer until very recently is not the whole truth. It certainly has truth to it, since we know from historical sources and older queer people who are alive today that it could be very, very difficult to be queer. It does not, however, mean that queer people never had what we have today: acceptance. There have always been and always will be queer people who found acceptance in their communities, and straight and gay people lived side by side as they do now. A happy lesbian couple living together as "companions" for several decades, or a cisgender mother in the 1880s giving her trans daughter dresses and teaching her embroidery are not stories we hear as often, but yet they are just as true.

Sources on queer people however tend to skew towards the negative side, and often they are not made by queer people themselves. For example we know that at least a 100,000 men were sentenced and imprisoned in the BDR for same-sex relationships, since we have the judicial records. We know of Eleanor Rykener, a 14th century trans woman from London, since she was arrested and we have her arrest record. What we don't always have and don't know are similar masses of public source material from West German gay men and lesbians who didn't get imprisoned, or written records from other trans women who lived in London in 1393.

We might have private correspondence, photos, and such, but that is often inaccessible and hidden away in people's attics. Or we might not even have that, as is the case for 14th century trans sex workers. What we have though, is some oral tradition on queer persons who have lived in their communities openly and nobody around them found that to be a bad thing as long as they did their jobs. According to oral tradition from my home country this was fairly typical in smaller rural communities: "he just was like that" is a phrase you see that people might use when describing a familiar queer person.

So, my point is that rejection of same-sex relationships has existed alongside acceptance. Most commonly people think it stems from the Bible, but others say those parts might have been added later or mistranslated. In what we perceive as "Biblical times", same-sex relations in themselves were not necessarily seen as bad, but i.e. ancient Romans deemed the receiving partner in sex between men to be "less than". It was, however, all right to be the top. Later interpretations and translations of Bible have had more part in this than the periods the books in Bible presumably describe.

In Europe, homosexuality has been generally condemned at least since the Middle Ages, when the story of Sodom and Gomorrah in the Bible was interpreted to mean same-sex sexuality, but not exclusively so. So-called "sodomites" could also include people who masturbated, or had sex before marriage, so the term wasn't really exclusive to homosexuals. In the Middle Ages, a concept for sexual orientation didn't exist, so "sodomite" and "sodomy" likely referred to all sexual activity deemed "ungodly" or "sinful" such as anal and oral sex, bestiality, and sexual abuse of a child.

On the other hand, the Orthodox church allowed so called "adelphopoiesis", lit. "brother making" until late 20th century. This was a marriage-like union of two people of the same sex, usually two men. The relationship was church-recognized and thought of as analogous to siblinghood. The Catholic church in Europe allowed adelphopoiensis as well until the 14th century. This is a prime example of what I'm going for in this answer, as it shows how these realities existed simultaneously, even though the interpretation of these arrangements remains controversial. Some historians have argued that this could have indeed functioned as an early form of same-sex marriage, and I believe that at least for some people it did serve that purpose.

The terms homosexual and heterosexual were coined by Karl-Maria Kertbeny in 1868 and started to be used mainly by academics. Homophobia certainly existed before that, but 19th century medical community was what made it more vicious. Kertbeny anonymously published his new terminology in a pamphlet that denounced anti-sodomy legislation in Prussia. Kertbeny, a queer person himself, was a pioneer and effectively created language for queer people to see themselves in and identify with. He also recognized a shared struggle that queer people faced in increasing anti-sodomy legislature and particularly its enforcement across Europe. By then the meaning of sodomy had also shifted, and it specifically started to refer to male homosexuality.

//e: Kertbeny coined the terms in 1868, not in 1848 as I had accidentally written