1) in the early days, it was often used as a 'gotcha' type thing by the church. so, if you're a philosopher that's talking about all kinds of crazy things like atheism, and you happen to be gay, they could get you on that when they can't get you otherwise. it was often used as a pretext to shut down opposition.
2) during various phases when the empire was weak, it was attacked because it was viewed as hindering population replacement. so, if there was a famine or a plague or a disastrous war, they'd go after the gays for not doing their job in helping to produce more workers and soldiers.
3) under feudalism and slavery, it was attacked because it hindered the ability for the serf or slave's owner to produce further offspring (which would produce more wealth, or be sold).
4) then bondage became replaced by debt, but the prohibition continued for the same reason.
oddly, now that population control (rather than population replacement) is the dominant goal of the elite, the oppression has mostly slowed down, except religious opposition - which is the same basic idea as we've seen throughout history: gay people don't create a new generation of donating church goers.
well, and also in russia. where the population is declining.
i know that's grim.
here's a document that explains this in more detail:
http://www.isj.org.uk/index.php4?id=310
