No one is attempting to force people to condone divorce and adultery, but there is a movement to force The People to condone homosexual marriage. And I am against laws criminalizing homosexuality.
Divorce and adultery are already legal. So why would making homosexual marriage legal force people to condone it anymore than having divorce and adultery be legal forces people to condone them?
The debate about homosexual marriage should be focused on what the impact on society would be of the change. I am unable to see a single person who would be harmed by making it legal, and many who would be greatly helped--really everyone would be helped, even those who are not gay, because of the strengthening of the institution of marriage by encouraging more people to enter into lasting meaningful relationships, and the resulting benefits that would ensue for all, the same as the encouraging of heterosexual marriage is good for everyone as well.
The only coherent argument against it is a religious one, and I do not believe the law should be based on a book that cannot be proven to be true. Until and unless the Bible can be proven to be fact rather than simply something that one must accept on blind faith alone, it should have no basis in dictating social policy.
That's not to say that the Bible can't still have and in fact does have a very positive contribution to make as a general guide for how to live one's life and make the world a better place in a lot of ways. But it is at the end of the day a book written by men and thus shouldn't dictate social policy anymore than any other such book.