Well, you see gay couples getting married doesn't hurt anyone while the rest of your examples do. A father and daughter getting married, then the kids are usually deformed, and that is hurting their child. Three men or four women, is polygamy which is illegal, because some of those in the marriage could be hurt emotionally by it. Also this is your religious belief with the insitution older then civilisation itself stuff. Seperation of church and state should always be remembered.

I don't believe in God, because there is no proof. And please don't say well there is no proof he doesn't exist, because under that same theory all criminals should just be sent to jail, because there is no proof that they didn't commit the crime, why don't we just forget about free trial? I refuse to believe in God until proof presents itself.