I hear what you're saying. But the problem goes way beyond gay marriage.
It does indeed. But I think one can very reasonably say--without being hateful--that allowing people to call themselves "married" who are overtly incapable(*) of becoming a mother and father--will make it harder to reestablish what marriage is really about.
(*) Much of society is dependant upon 'polite fictions' to lubricate the social fabric. Even if a husband and wife have no intention of having children, one can still plausibly believe that they might nonetheless become mother and father. Such a belief, however, becomes untenable with same-sex couples.To use yet another analogy, there is a tendency that property which is severely run-down or distressed tends to attract more vandals than property which looks like it's actively maintained. Someone who is in a park that's absolutely prisine may be averse to littering, but the same person, if in a park with piles of litter all around, may have little qualm throwing more junk onto the piles. If the park is clean except for one candy bar wrapper and there aren't any trash cans nearby, a mildly-well meaning person might hide it. This wouldn't solve the problem of the existing litter, but it may help avoid further litter.
Polite fictions can't provide structural strength to society, but they can help stem exponential breakdown. Unfortunately, by making society's "litter" less obvious, polite fictions sometimes cause people to think they can litter without being noticed. Unfortunately, such people, while often caught, are seldom contrite.