As noted, the population thing is complete bunk. More gay couples will not mean less straight couples, and marriage of either does not affect the other.
Regarding the 2nd point, this would also apply for couples that marry but don't have children, and couples that marry and then divorce. If you're willing to apply the same penalties for these two groups, then I'll concede that your argument holds from a utilitarian standpoint. Except, you'd also have to admit that gay couples that have children (but made from a surrogate) and gay couples that adopt (who save us far more money than couples gay or straight that have children of their own) should enjoy the same benefits as straight couples with children, making your argument ultimately futile.
What you should instead do, if you want marriage to exist at all while maintaining at least the illusion of basic human decency and equality while maximizing utilitarian efficiency, is scrap the entire idea of marriage and instead make tax benefits based on children, given to the parents and their spouses.
Regarding surrogation, my gay best friend, a girl that I used to work with, was discussing the idea of childbirth. She's in a committed relationship with her girlfriend, and she told me that, were they ready for children, they both would want me to be the father for my genes. Feels good man. I don't know if I'd ever go through with it.