The ability to reproduce. The government has no compelling reason to care if Steve and Jim stay together in a committed relationship. BTW, if you really know the gay community, gays refers to actual gay marriages as 'mythical', because it is so rare in the gay community that two partners stay together very long. A study puts the length of these gay 'marriages' at 1.5 years vs. 7.2 years for a man and woman marriage.