The Essential Properties of Marriage

In Catholic marriages, you make certain vows, including vows to be open to receiving children. If someone doesn't mean those vows, there's going through the motions that they promise, then that can be grounds for an annulment. But from a purely natural law perspective, as long as a couple has unity and ideal in dissolubility, they promise, at least until death do us part, then they have a good and natural marriage. Even if you don't end up having children, marriage involves the monogamous union of a man and woman engaging in the act that is designed to create children - something that cannot happen with the same sex couple.