Same-sex marriage also promotes family life
It is fallacious to say the children will grow up without knowing their (biological) parents — sperm donors and surrogates help couples, gay and straight, to conceive, knowing they won’t have a relationship with the resulting child, because it isn’t their child, it is the child of the parents who raise it.
Additionally, the children have a family history, since they are planned for, and wanted, by their parents before they are even conceived — they are born right into a ready-made family.