In a strange sort of way; they are saying that Lisa cannot be the mother; even though she is a woman. And she is obviously not a father; so how can they say she is which is illogical at the very least?
In a way they are talking traditional marriage; because it seems they are saying a child can't have two mothers(two dads). Kind of burst the balloon in their theory of two gays raising a child as two parents. I call it stupid logic.