That's certainly true. The question is, do homosexuals have the right to force others to approve of, accept and "celebrate" such relationships.
The libertarian answer is to get government out of the marriage business. Were that entirely done, I'd have no problem with homosexuals "marrying" or whatever they wanted to call it.
Of course, the libertarian solution also allows me to disapprove, not accept and condemn such relationships. I would also have the right to discriminate against those who shove unpleasant personal characteristics in my face - or even if I just didn't like them personally. Bar owners would be equally within their rights to put up "No Fags" or "No Breeders" signs.
But in the current situation, until we achieve a more libertarian society, allowing a fraction of 2% of the population to redefine a social institution of several thousand years standing is just plain silly.
BTW, any logic that justifies changing marriage to include two same sex partners is just as good when applied to multiple partners and non-human partners.
Read the Canadian court decision on this subject. The convolutions they go through to justify two same sex partners are clearly straining at gnats while swallowing camels. Pop out the two same sex partners and pop in a boy and his dog, my brother & three sisters, me and my pet rock - whatever - and it reads just as well.