Mary has been grazing on loco weed and has stumbled around and lost her moral compass.
However you see that as a "choice" or almost beyond choice, it still is the case that existing rational conventions around marriage are oriented towards the one man and one woman union that is our very strong custom. I see no reason to change it. Let us be tolerant within reason, and let us make sure appropriate contracts can be entered into. It is not a civil right to force all laws and persons to erase differences between your own relationship type and the legally recognized one.