I won’t address this argument comprehensively right now, but I do want to point out a few things:
(1) The “vague sense of fairness” is a neat slight of hand. Same-sex couples are being something a lot more substantial, including societal dignity, and very tangible legal and financial benefits, the denial of which causes harm on a level much higher than a vague sense of unfairness.
(2) The presumption of a lack of sexual fidelity in homosexual couples is a neat trick, especially since the history of American jurisprudence has over the past century been on a track of getting its nose out of the bedroom. At one point, the issue of marital fidelity for married heterosexual couples was the subject of a lot of law, but we have as a society decided that this kind of thing is best left up to individual choice rather than legal boundaries. So sexual fidelity is more and more not a business of the government, and this should be no different than in the context of civil marriage.
(3) The assertion that the fate of “step-children” is inferior to that of others is thrown out there without much support and more importantly without any indication of how it is actually applicable to same-sex couples, even ignoring the fact that same-sex couples are often in a position to adopt children, which is actually good for society.
(4) This entire argument rests on a series of assumptions, wishful thinking, and outdated societal attitudes. All the evidence we have today is that more and more people are accepting of same-sex unions, more accepting of civil recognition of same-sex marriages, and that same-sex couples are able to create family conditions in which children thrive.
(5) I believe that a few years ago, statistics revealed that in the United States, the number of childless adult women was now greater than the number of women with children. Furthermore, add to that divorce, remarriage, out-of-wedlock births, and other societal trends show that broad policy such as marital policy should no longer be based on assumptions that marriage, coupling, and families are primarily for raising children that belong biologically to the two adults heading the household.