The homosexual male has been used to a lifestyle of promiscuous sex and lots of it. The exclusivity of marriage does not fit with the promiscuous lifestyle. And that exclusivity has one purpose: to shield the other partner from venereal disease. If the promiscuous lifestyle continues into the same sex marriage relationship, it will become a vortex for disease that will be passed on to all it comes in contact. It will be mother nature’s punishment for ignoring its reality.
Every gay couple I know, married or not, sees adultery as a given.
They want marriage for acceptance and the benefits.
Historically I think a more important reason for the exclusivity of marriage was the man’s desire to know which children were really his—if his wife was not faithful to him he might be raising some other man’s children and passing on his inheritance to a son who wasn’t really related to him. The danger of venereal disease was probably much less a factor before 1493, when Columbus and his men returned from discovering the New World and brought back syphilis.