On what basis should Christians oppose child marriage?

Indeed, AFAIK the Bible doesn't make any overt statements on this subject. It is, however, usually accepted that Mary was around 14 when giving birth, and was married at that time. read more

One reason would be that nowadays much child marriage is forced marriage. It's one thing for a marriage to be arranged -- not to Western taste, but if both principals consent to it, it can be decent enough. read more

Gay marriage is a relative newcomer to the political arena, and the fight against the legalization of gay marriage is not yet a "lost cause" in the eyes of some faiths. Another reason that people of faith oppose gay marriage while turning a blind eye to divorce is that many religious people view homosexuality as a particularly heinous sin, worse even than adultery or fornication, and most certainly worse than divorce. read more