Point of fact, just for the record... In places where all the focus has been more on forcing different-sex marriages and forcing many people to marry or even, to have children 'for the sake of it' as it were... There have long been lots of marriages of gay people with different sex partners and some of them even produce children.
So if one were to be very cynical and forthright about it, it could even make a certain kind of sense for the proponents to argue that keeping gay marriage off the table does have certain benefits. That is, IF you are speaking solely in terms of the raw chances for general biological procreation. Then again, so would banning abortion -- but what the hell else one gets in those scenarios is another question entirely.
Of course, that wouldn't prove that it's obviously any more beneficial for children or family stability to only allow different sex marriages. That would be another whopper of a claim to attempt to find supporting evidence for. And there is also the huge lingering problem of, what other costs the singular emphasis on different sex marriages has brought on in places where those have been shotgunned onto most of the population with few or no other socially supported options.