They cannot by themselves, but can be open to children with science. Gay marriages open to children are not invalid.  Indeed, these are often second marriages where one or both spouses already has kids from trying to pretend to be hetero - a stupid errand.

There is an impediment for non-functionality in sex.  Gays are functional.

Catholic gay marriage is inevitable.  Indeed, when I took marriage class, both in High School and in pre-Cana, we were taught that the couple performed the sacrament, with the priest witnessing but it was a sacramental marriage regardless.  Gays can do that. Their families will demand that and gay priests will quietly cooperate until the generation still resisting (meaning you) die off or are small enough to ignore. Francis is not appointing anti-gay bishops and is letting the ones we have retire at 75.  It's a new day.