Sass
The historical purpose of marriage has little to do with sex - as such.
The purpose of marriage was to restrict and control the transfer of property, to control inheritance. Hence, the first duty of a wife was to produce an heir so that property could be kept within the "bloodline" the easiest way to do this was to control the sexual opportunities available to the wife. Two men together or two women together could not produce an heir.
The nasty hand of religion only came into marriage at a rather late stage when it was used to effect social control. Credulous people (and there are still some around) could be frightened into submission by being told they were offending the King of the Fairies in the Sky.
What other people choose to do with contents of their underwear should be of no interest to anyone else. And if two people want to celebrate their affective relationship by making a public declaration of this relationship then why should anyone else, including the King of the Fairies, be in the least bit bothered?