And using a given denomination to claim "Christianity is against gay marriage" when there's scores of denoms for it makes sense how?
Can show you however many denominations and sites you want saying what carm did. Pleanty of Jewish ones too.
Bible doesn't mention masturbation (or due to the word's nonexistance thousands of yeas ago,) the equivilent. 'Sin of Onan' wadsn't maturbation but coitus interuptus. And if you're not Jewish or claiming to be bound to Jewish law, 'spilling your seed' doesn't bind you any more than any other Jewish law would.
Christianity itself is on the whole anti-sex and pleasure. It's main figure, as well as main authority fiugres were supposedly celibate (though Jesus' tomb was supposedly discovered with Magdalan his wife, and their children.) Afaik Paul was actually celibate, at least when an Apostle. But when your authority figures are celibate and one (Paul) says it's better to not touch a woman, get married, or have sex even if married, then that religion isn't going to be anything but anti-sex.
Interestingly, this anti-sex view isn't supported by the first half of the Bible where God says 'be fruitfuland multiply' and other commandments command men to satisfy their wife's sexual needs.
Not to withhold food, clothing or conjugal rights from a wife (Ex. 21:10)
So where Paul got his thing from is a good question. Certainly not from God.