Religions have a history of trying to keep the agonies of marriage between a man and a woman through legislation. This isn’t just a selfless act of kindness you may expect from organisations that regularly convince their poor, stupid members that tithing to them is a great idea.
Most married people will denote their status with a ring, necklace or the dead eyes of a crushed soul, yearning for freedom.
So when the religious priests go out on the pull, to gay nightclubs, pub car parks, airline cabin crew training courses etc. they can look for the rings and steer clear of the breeders who are not worth talking to.
If the gays (or to use the correct term, “the gayers”) were allowed to marry, those religious types would have a harder time to identify cuddle buddies, and probably just head for the first person under 12 they saw instead.