From the comments:
Mrs. Gallagher states "...sexual passion between men and women creates children, and those children do better and society does better when the mother and father who make them raise them together." I completely agree. If a man and woman make a baby, it would be best if they both raise the child. No one is arguing that point, except for Maggie. How a married same-sex couple is able to interfere with that scenario is anyone's guess. The rationale goes something like this: If same-sex couples can marry, then one should assume that heterosexual couples will no longer be able to marry, procreate and raise their children? Isn't that the basic argument that Maggie is trying to make?