Maybe because there is an injured party in your examples of murder and buggery. 2 gays marrying has no injured party, no ones rights violated.
I suppose it depends upon what you want your society to look like. My understanding of the gay community (based on observation of gays we've known here and elsewhere) is that sex is recreation and partners are the current convenience up to a certain age. That certainly resonates in our current culture where adultery has been celebrated on stage, screen, and in politics (as long as you belong to the correct political party) and is being inculcated into our culture through continued exposure. The idea that two gays marrying, then divorcing and remarrying, mirrors the current state of heterosexual marriage is just sad.