I see nothing wrong with a homosexual marrying someone of the opposite sex. Except that the marriage partner would have to be marrying for something other than sexual love. I see no reason in the world why homosexuals cannot live a life "in the closet so to speak" and continue to allow "normal heterosexual society" to dominate and form the opinions of mankind.
When you say there is no evidence of homosexual embracing societies faltering, you overlook several societies in the past that did embrace free homosexual love and did indeed falter. My personal experience suggests that homosexual behavior includes hitting on young straight males for recruiting purposes. It is this behavior that needs to be controlled. Permitting homosexual marriage will only foster more homosexual behavior. A change in society that society certainly should have a healthy debate about first.