So you have identified a difference. Being infertile is a naff red herring brought in when people can't face admitting the facts. The point is about potential. When two people try to have children generally speaking they have no idea whether they will get lucky and clinch the deal. However, a straight couple has the potential to get lucky. A gay couple does not. Their equipment is not complementary; they will simply never be able to have children with each other. That is a difference; it is THE difference.
Prince Charles & Camilla had kids while married to other people, but are too old to have kids married to each other. It didn't stop them being married and it won't stop him being head of the Church of England when he becomes King either, so the procreation angle is as much of a red herring as the religious angle on this matter