If Catholics oppose same-sex marriage as sinful why do they not also oppose divorce as sinful?

Answer 1

You're talking apples and Oranges. In point of fact, both of these things are sin, but of a different variety. Catholics are against divorce because a true sacramental marriage between a man and a woman lasts until death and there is nothing that anyone can do to separate them, a civil divorce may be allowed in certain extreme cases (abuse, etc.) but there is no actual divorce, they remain married, just living apart under a civil divorce.

"Gay marriage" is an entirely different thing. It is rooted in the sin of sodomy which is actually sacrilege, the sin of sodomy is one of the four in The Bible of the "sins that cry to heaven for venegance" (along with murder, oppression of the poor and defrauding workers of their due wages).