The yelling and smacking is where I think the family is in the wrong here.
They have absolutely every right to not approve or support him if they so choose and he is 18 or older. I'd probably distance myself from my son if he was gay, and if he was 18 or older, he'd need to move out and get his own place. I wouldn't yell or become violent. He has the right to do what he wants with his life after he's 18. And I also have the right to distance myself from him like I would anyone else I don't want to associate with.
But the screaming, and more importantly, the smacking, is totally uncalled for. Histrionics never help anything.