1) Marriage promotes the positive rearing of children.
2) Marriage promotes a stable economic situation.
3) Marriage promotes a more stable social situation.
4) Marriage promotes increased physical/mental health.
There is plenty of research that shows that married couples produce healthier children, better economic and social situations, and improved mental and physical health.
So, now we know why government supports marriage, and we know that marriage improves the health and welfare both of the participants and their offspring, the support for gay marriage comes from the fact that it meets all of the above criteria. We also know from research that gay couples rear children as healthy as those reared from straight couples, and gay couples are proportionally as well off economically, stable socially, and both physically and mentally as healthy as their straight counterparts are vs. singles from each orientation.
With the congruence of these issues, and the demonstration that there is no difference between the benefits of straight and gay marriages, there is no legitimate argument to NOT legalize gay marriage...as long as one goes the route of THIS course of debate. This is the "Family" debate.
The only legitimate rebuttal to this is for the government to get out of the marriage business altogether...which in a sense, is a different debate altogether.