You know, the same kind of problem that a person who doesn't understand basic human sexuality (men have sex with women and vice versa by design) have.
I didn't see Michealangelo sculpt the Pieta. I haven't seen firsthand evidence that anyone, he or anyone else, did either. Yet I have a very difficult time seeing that masterful work as the result of the actions of water and wind on marble, for some reason.
But whether the design was that of a creator or the much less believable lightning bolt and primordial swamp notion isn't really the point. Human men are biologically suited to have sex with human women.
And I note with interest that nobody has produced the multiple historical interests of homosexual marriage over many cultures and times. Anyone? At all? No? All right then, quit the bullshit about this just being another evolution of marriage, m'kay. Whether women were viewed as chattel or cherished helpmeets, men as owners or loved husbands, and whether marriage was a childless couple or polygamous mess, it has always, always, always been between a man and a woman. Period. And it always will be, whatever you reality challenged gomers pass into law or don't.