"Certainly not. 'I love my daughter but don't endorse gay marriage' would do just fine."
I would treat it as any other sinful lifestyle. If one of my adult children came to me and said "I've taken up practicing witchcraft." I wouldn't disown them, but I would certainly let them know that didn't approve of their lifestyle, nor would I attend any ceremonies or allow it to be practiced under my roof. The same with homosexuality. I would be doing a disservice to tell them, "Oh, that's just fine! I'll support you no matter what!"
Some people are genetically more inclined to become an alcoholic(me included)
But my dad never encouraged me to become one.