State recognition of marriage exists under the notion that encouraging stable parenting hand in hand with procreation is beneficial to the state. The state doesn't grant special status to married couples to encourage romantic love, but to further the interests of stable procreation. It's obviously not a perfect system, but it does encourage it. If romantic love was the sole criteria for a marriage license then the notion of marriage would become meaningless (polygamy, etc).
While some straight couples are unable to have children, most can. The process of weeding out which couples are sterile, whether just one member of the couple is, whether they are permanently sterile or simply have a very small chance of bearing children, etc, would be a very costly and time consuming process. In addition to which, straight parents adopting children is a benefit to the state. While (as far as I could find, if y'all can find some I'd be interested in reading it/them) there are no scientific studies showing that gay parents are better or worse than straight parents. Which means the burden of proof lies on the people desiring special legal status, because if everyone automatically received the legal recognition then it wouldn't be a "special" status anymore.
Gay couples on the other hand are 100% unable to have children together. Again, as far as adoption I couldn't find any - but will happily read them if y'all find them - studies showing that gay parents are better or worse than two straight parents or foster parents or straight single parents.
The question isn't what behavior the government should regulate, but under what circumstances should the government grant special legal status to someone?
The answer, as I understand it, is when those people provide a benefit to the state. Corporations get special protection from liability because limited liability means more growth, etc. Marriage is benefical to the state because it encourages (not guaruntees) stable parenting and procreation.
Straight marriage has proven benefits for the state, and I'm interested to hear what benefits y'all think gay marriage has for the state? Because my understanding of the law is that unless someone proves that the government granting them special legal status provides a benefit to the state then they don't deserve the special legal status.