I made this claim yesterday. It has occasioned some misunderstanding, so let me elaborate.
One misunderstanding is that when I say that marriage entails submission and sacrifice, I am contending that homosexual couples are selfish. I did not mean to suggest that homosexual couples are incapable of submission or sacrifice; I am sure that that happens in many actual homosexual couples. My thought is that the complementarity of man and woman entails (mutual) submission, in a way that the similarity of a same-sex couple does not. Likewise, parenthood entails self-sacrifice, and parenthood is entailed in the social institution of marriage. Some actual marriages don't have children, and some homosexual couples do, and no doubt there are many exemplary marriages and parents contained in those groups. I am talking about is the social ideal, not your relationship or mine.
Another misunderstanding is that when I say that the social purpose of marriage includes children, I am condemning childless marriages. Again, I am talking about the social ideal, the reason that society depends on marriage. All actual marriages, mine included, deviate from this ideal. I do think that marriage as an institution entails children. Children are why complementary couples take the step to become a permanent, faithful, one-flesh unit – that is, become married. Some actual couples do not have children. Society, and the institution of marriage, can live with that. However, if all children were produced without marriage, society would disintegrate. And if no marriages produced children, society would end.
A third misunderstanding is that I am ignoring love as the basis of marriage. I do think that the deepest love among human beings is found between parents and their children, and between husbands and wives. Marriage, though, adds to that love a permanent institution that changes husband and wife into one flesh. This relationship, unique in all social life, has deep consequences for their material lives, as well. Sometimes the feelings of love fluctuate, especially between the husband and wife. The marriage does not need to end, though, if love is at low ebb; rather, the institution of marriage is there to protect the couple from their changing emotions, and especially to protect their children from rash emotional actions that their parents might feel like making.