I think "equal before the State" is implied here. After all, gay couples can already perform marriage rituals and live together, what they lack is the recognition of the official institutions.
Regardless of what the state thinks, unplanned pregnancies are the domain of heterosexual partnerships. In monogamous homosexual marriages, children come about intentionally or not at all (exceptions here only prove the rule).
Point being, the outcomes of straight marriages will inevitably include more children, so it's in the interest of the State to make sure sexually-active heterosexuals marry, marry well, and stay married. There may be other goals to consider, but I'm a bit surprised that biology and human nature haven't come up in this discussion yet, especially considering recent trends in the extramarital birth rate.
