The absurdity of 'interchangeability'

Of all the ridiculous conservative arguments, this one always drops my jaw:

How, exactly, does same-sex marriage speak to interchangeability anymore than different-sex marriage? In every form of marriage, a man or a woman joins with a man or a woman because either a man or a woman is the gender to whom he or she is drawn. Yes, one can be attracted to both genders—but again, bisexuals might enter into a same-sex or a different-sex union. None of it speaks either or in support or against marriag...