I suppose homosexuality is abnormal in a completely harmless way. The norm is the most common, the usual. A cat with no tail is therefore abnormal, but its lack of a tail does not bother the cat, or interfere with other cats. Something that is unusual is described as abnormal but that is not a criticism of it. That is why the use of the term “hetronormative” as a disparagement makes absolutely no sense.
Homosexuality in most societies in the ancient world was not considered an problem. Gay people then as now were only a tiny percentage of the population so population replacement was not an issue. I don’t know why some people are gay. It certainly doesn't seem to be inherited because although some gay people do reproduce, there seems to be no greater chance than usual that their offspring will be gay.
Given that Christianity was basically a construct of the Roman Empire, I have no idea why it became so fanatically anti-gay or what purpose its hatred of homosexuality served. The Romans for most of their history didn't care whether someone was gay or not. Maybe it came from the fact that by the time the empire had adopted Christianity it was already in sharp decline, and population replacement had become a serious problem. In fact the Emperor Constantine believed that Christianity could be utilised to increase the native Roman population. After the collapse of the empire Christianity was also in crisis for a while and Christianity was obsessed with pressuring people to making as many little Christians as possible. That is just one half-baked theory.
I just don’t see why it is even an issue today. Live and let live is my attitude.