This sounds like a straw man (and one that I often hear in the gay marriage debate):
The anti-gay-marriage argument that simply makes no sense to me is the one that says allowing gay folks to marry will mess up my marriage – my heterosexual marriage. I don’t follow the reasoning that gay married couples will undermine the ability of straight married couples to form and sustain marital partnerships.
Perhaps someone has made that argument somewhere, sometime, but I’ve never seen or heard it myself. It would be helpful if she would provide a link to support the straw man. Of course it makes no sense to her. It makes no sense at all, which is why few people make such an argument.
I think that this may be a perversion of the real argument, which is that, for those uncertain of their sexual orientation, it will weaken societal pressures to have a heterosexual lifestyle and marriage. If society is no longer heteronormative, then a little boy might grow up thinking that it’s OK to marry his friend Joey, instead of Sally. Actual homosexuals are going to grow up to be gay regardless, but it’s not necessarily a good idea to encourage wavering where it exists. Now, one can argue whether it’s a good or bad thing to do so, but that’s the argument to be discussed..
The argument isn’t about existing marriages–that’s nutty. It’s about future ones.