"These people are getting to pay less by having their 'marriage' recognized by the USA. That means my taxes have to go up!!!111"
Something like that.
Not saying it's a good argument, and I'm not saying that MM would use that language, just relaying what I think MM is getting at. So, if things are zero-sum (which I don't think they are), then wide recognition of gay marriages could affect the tax base.
FWIW.
Cheers,
Scott.