Because that's the core reason for opposition to gay marriage.
There's no rational reason, either data-driven or scientific, to oppose gay marriage. Homosexuality occurs in hundreds of species, and it's evolutionarily important.
I disagree with the last statement. Evolutionary important? How?
Now I don't have anything against gay marriage, but I don' see anything useful in "gayness" as a whole that's worth preserving. No offense to anyone, k?
Kin selection. Inclusive fitness. It works on a similar principle to the eusocial principles that drive ant colonies, or alarm calls in ground squirrels, or nest-helping behaviors in young Florida scrub jays.
The idea is that you get
more virtual offspring by helping out your parents and siblings than you get offspring by trying to reproduce yourself. Since you're not using up resources for your own kids, more of your siblings and siblings' children survive, and you can help them grow. Since your siblings share half your genetic material, and their kids share a quarter, you're basically getting free 'virtual offspring' by halves and quarters if you help them out.
The relevant equation is rB > C. Homosexuality will be favored whenever the fitness benefit of homosexual behavior (B) multiplied by the coefficient of relationship with the behavior recipient (r) is greater than the fitness cost to the organism.
Transsexuality is also natural and important in many species.
There are a number of other major reasons, too.
Read this for a good overview.