How does an infertile heterosexual marriage differ from a same-sex marriage?
One big difference is that the heterosexual couple enjoys sexual complementarity, and the fullness that brings into their relationship, even if they cannot have children. Though the situation of an infertile couple is very different, there is a disturbing parallel to same-sex marriage in the situation of couples who simply choose not to have children.
Such couples are still able to have sex, the fullest physical expression of love between husband and wife. But they are doing something that profoundly disturbs the nature of the sexual act. Sexuality has two aspects: the procreative (bringing forth children) and the unitive (strengthening the union of the couple). Artificially separating the unitive from the procreative brings discord to a marriage, distorts the relationship between husband and wife, and ends up harming their unity as spouses.