I mean, really, there is no principled reason if marriage is about "love" as those that want gay marriage advocate- then there really is no reason not to extent "equal rights" to multiple partner relationships (in fact a very prestigious paper by the leading LBGQFTFC scholars publicly called for this), animal relationships, open relationships (ie no standard of monogamy), the my strange addiction relationships on the TLC show and a hole host of interesting combinations. Those people are also born with their sexual proclivities and should have right to have them recognized.

People that want gay marriage should just be open about it: they dislike the original institution, feel it is sexist/mysoginist/wrong and want to change the institution permanently and in a massive fashion. It is not just a marriage with the general traditional principles of monogamy transplanted to gay couples. It is a entirely new formulation and it is disingenuous to sell it as a the same institution for a new group. Their scholars and intellectuals have said so openly, although primarily in academic circles.

Additionally people need to be honest this does have large effects from schools to non-profit status. CA is already attacking religious and conservative organizations' non profit status for refusing to accept gay marriage/having different stands on homosexuality.

Disclosure: I'm not religious and mostly libertarian about this issue.