The thing I have never quite understood, is that most attitudes against gay marriage seem to be based somewhere down the line on Biblical teaching.  While the Bible states clearly (in the new testament at least)  that marriage is between a man and a woman, it also mentions several other things that are considered perfectly OK and not controversial at all in most western societies such as divorce and remarriage (when both partners are still alive).

Can someone explain to me why these are OK but same sex couples getting married are not?