Who said anything about preventing heterosexual couples from marrying? That's an odd concept.
It was implied in your comment about encouraging the propagation of humanity or whatever nonsense it was. You were implying, intentionally or not, that allowing gay marriage would somehow reduce the rate of heterosexual marriage, or reduce the general birthrate of a society. Both ideas are laughably absurd.
Since you do not believe that government licenses marriage to provide a good structure for the raising of the next generation, why do you think they license marriage?
It's a vestigial legal framework based on religious practices. But as long as the government is involved in providing legal rights to people, it is obligated to provide those rights equally.
I believe it is prudent for society to invest money (tax breaks etc.) in an attempt to propagate a future. I see no reason for society to invest similar $$$$$$ simply to make homosexuals feel good about themselves. So that they can say, "see, ours is simply an alternative life style, of equal value to the community."
I believe it is the government's role to defend our rights as citizens and human beings. What does being straight have to do with propagating a future? Gays can't reproduce? Gays can't adopt? Gays can't have a family?
I hate to break it to you, they can. They can do all those things. And it is our obligation to extend to them the same legal rights that straight couples have.