What Christianity is and is not is disputed territory. That there exist passages proscribing homosexual acts in Christian texts is undeniable - and a problem for a lot of Churches that would rather those passages were not there.
Probably right there:
Leviticus 20:13
“If a man practices homosexuality, having sex with another man as with a woman, both men have committed a detestable act. They must both be put to death, for they are guilty of a capital offense."
But the same went for adultery between a man and women. Incest too. There are loads of rules that Christians aren't restricted to by the old testament. E.g you couldn't be a Levite unless you had pure Jewish lineage it says in some places. Whereas Christianity is open to gentiles i.e non-jews. But then again we have in the New Testament saying:
Romans 1 : 26-27
"Because of this, God gave them over to shameful lusts. Even their women exchanged natural sexual relations for unnatural ones.
In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed shameful acts with other men, and received in themselves the due penalty for their error."
I suppose the word 'natural' is the key. Nowadays homosexuality is understood to be perfectly 'natural'. Again I think this goes back to adultery as sin. 'Natural' might mean 'monogamous' or even 'loving'. Which would make the idea of the gay community not being allowed to marry contrary to Christianity.
Then you have the whole 'do not judge', 'salvation through belief in Jesus rather than acts' (which is another discussion), and the commandment to "love thy neighbour".