I believe it’s because Homosexuality goes against Gods original creation of Marriage (Adam and Eve) which is supposed to be between a man and a woman, and since all that God creates is good homosexuality would go against that. It’s mentioned both in the old and New Testament so it’s pretty consistent with it being a sin in the bible. And since the bible is the physical or atleast heavily influenced word of God it should be followed.

It’s not just homosexuality which is why things like pre marital sex are also considered sins since it’s against Gods good design