Lots of supporters of gay marriage affect an objective or even-handed position, when they are really just shilling for a special interest for the group with whom they sympathize. It is not really about "equality" and personal definitions and choices (since they are trying to strip everyone else of the right to choose to recognize their role-playing games as real or refuse the same), it is about obtaining a special privilege for homosexuals, which they agree with that group obtaining, whether out of sentiment or personal interest.

Marriage is a thing. You don't change its nature by adding a word to the label and making it appear that the true nature of the thing is actually only as aspect of the real nature. That is the view of proponents of marriage. The proponents of alternative domestic arrangements have their own agendas and are as beholden to them as anyone else. They will not be any more fair or impartial, because their reasons for their choice are just as personal and egocentric as anyone's.