I know that at different time throughout the world people seemed a lot more cool with gay people, E.g. ancient Greece and precolonial America.

I can't say anything about pre-colonial America, but I can say that ancient Greece seems to be widely misunderstood on this issue. A lot of people, it seems to me, look at pottery of, like, three guys apparently engaged in an orgy and then conclude that ancient Greeks must have been broadly OK with homosexuality (or at least homosexual behavior).

My training as a historian concerns the history of philosophy. I'll be the first to admit that my knowledge of "life on the ground" in, say, classical Athens (let's say: around 380 BC) is limited.

Much of what I will say uses some of our familiar understandings of sexuality and sexual orientations, but as you can imagine, sexuality was conceived very differently in ancient Greece. For an answer on this, see: https://www.reddit.com/r/AskHistorians/comments/7lnrh5/how_common_was_homosexuality_or_what_wed_now_see/

But I do know the history of philosophy well, and I can say that there was tremendous push-back from some intellectual corners regarding homosexual behavior. (The above answer says that sexuality in ancient Greece was defined less by choice of partner and more by role. Maybe that's true, especially outside philosophy, but as we shall see below from Plato, whether you're sleeping with a man or a woman determines whether the action is right or wrong.)

For instance, Plato in the Laws says that the pleasures that come from sex between two men or two women are wrong because they are "unnatural." Specifically, this is what he says in the first book of the Laws:

"Now the gymnasia [i.e., physical training] and common meals do a great deal of good, and yet they are a source of evil in civil troubles; as is shown in the case of the Milesian, and Boeotian, and Thurian youth, among whom these institutions seem always to have had a tendency to degrade the ancient and natural custom of love below the level, not only of man, but of the beasts. The charge may be fairly brought against your cities above all others, and is true also of most other states which especially cultivate gymnastics. Whether such matters are to be regarded jestingly or seriously, I think that the pleasure is to be deemed natural which arises out of the intercourse between men and women; but that the intercourse of men with men, or of women with women, is contrary to nature, and that the bold attempt was originally due to unbridled lust."

There are a complete of remarkable things about this.

The first is that this argument is very similar to the one that you'll hear from conservatives in the West too: that homosexual behavior is wrong because it is an unnatural use of our bodies. And this makes sense, because Plato is relying on the same sort of understanding of the body. (Notice that the contemporary argument doesn't rely on the Bible, or anything like that: it draws this conclusion based on what is natural to our bodies.) Just like many people who oppose homosexual behavior today, Plato thinks that our bodies were created by someone who intended each part of our body to have a natural function. (Philosophers refer to this as a teleological understanding of the body.) We can use our bodies in any number of ways, but there are certain ways that its designer -- God -- intended it to be used, and when we use it against these intentions, we are using it unnaturally. Plato spends a long time discussing in the Timaeus the function of each part of the body. For instance, the eyes are created specifically so we can look upon the heavenly bodies. The lungs exist as a cooling system for the heart, which tends to heat up.

The second remarkable thing is that Plato thinks that homosexual behavior will cause "evil in civic troubles"......... somehow. He doesn't really elaborate as much as we'd like him to.

The third remarkable thing is that Plato as an author spends a lot of his time in various writings depicting men basically salivating over each other's good looks, and sometimes even depicts men trying to seduce each other. I am not sure I am going to put this point as precisely as I'd like, but I think of it like this: Plato is individually homophobic (i.e., he is against homosexual behavior) but culturally he is not homophobic. What I mean is that he has no problems with works of art that depict homosexual behavior or sentiments, but he thinks that the act itself is unnatural and wrong. There is no sense that Plato would have been opposed to the pottery we have that depicts a bunch of men all having sex with each other.

It is also worth pointing out that Plato often reacts very negatively to the common ancient Athenian institution of pederasty, by which an older man would mentor/tutor a younger boy in exchange for sexual favors. (I am making it sound kind of transactional, which it was, but it was also a cultural rite of passage.) He doesn't think that sexual behavior belongs in someone's education/edification. He and his teacher, Socrates, strongly and vocally abstained from pederasty, but it isn't explicitly related to his criticism of homosexual behavior, though. For instance, Socrates denied ever having anything to teach anyone, and therefore he couldn't be a teacher. And also there is a strong sense that bodily pleasures actually frustrate someone's edification and get in the way of their learning.