The institution of marriage broadly exists for all men and women, because men and women can potentially have children. This is a social construct that exists because civilization cannot exist without a constant supply of people. We protect marriage and make it special because it is an investment in the future of humanity.
NOT EVERY MARRIAGE needs to produce children for the institution to be valuable. NOT EVERY MARRIAGE will produce happy, well adjusted, productive citizens. Still, the institution works fine, because it produces enough.
Gay marriage has zero potential to produce children. None. They confer no potential benefit to society.
So, a million hetero couples could produce five million new people.
A million gay couples would produce none.
If some hetero couples produce more or less than others, no big deal, so long as a sufficient supply is kept up. Letting all men and women willing to accept the responsibility of marriage do so strengthens the institution. In some cases it won't pan out in the short term, but long term, keeping marriage special, honored and protected is worth it.
Gay marriages will in no way prevent heterosexuals from having children, both in and out of wedlock as they now do. Not will it prevent heterosexuals who wish to marry, stay together and raise children from doing so. Thus your argument is unsound.