Other than the philosophical argument, there's the political argument you have brought up.

The only reason the state cares about marriage is to encourage population growth. Since gay marriage cannot contribute to population growth 100% of the time the state should not give them any benefits. There is a weak argument that some normal married couples don't have kids or can't have kids. But these are the exceptions to the rule and there is nothing preventing those who choose not to have kids to decide to have them the very next day.

Marriage as a recognised union is important for reasons outside of politics and philosophy. In business, especially finance and real estate, marital status can be a very important factor in whether or not money will be loaned, a house will be sold etc. When businesses hire women they will favour a woman that is already married and already has children because the chances they will take time off is significantly less than than that of a childless single woman.