Why would a same sex marriage not include monogamy? And why would it be less demanding. Otherwise, you refer to propagation which reduces relationships to nothing more than the breeding of livestock. My desire to have children had nothing to do with benefiting society.
Furthermore, none of this addresses the issue that same-sex couplings cannot be considered as being of the same value as the real thing. Devaluating once marriage through things like infidelity is an individual choice that many heterosexuals make.