You don't seem to understand: they can be rational and maintain that a relationship between two people categorically incapable of producing children together bi

You don't seem to understand: they can be rational and maintain that a relationship between two people categorically incapable of producing children together biologically, two people of the same sex, can’t be a marriage. That's explicitly not rational, unless you are going to deny marriage rights to opposite sex couples who are "categorically incapable of producing children together biologically."