(Nota bene: "Marriage equality" is really part and parcel of a larger goal, that of "orientation equality".)
All parenthood becomes a legal fiction. If same-sex couples must adopt to have children, then heterosexual couples must as well. Anything else is invidious discrimination and, thus, wholly repugnant to Public Reason.
The scrapping of Paternity Laws. If homosexuals can enjoy a one night stand without fear of a Paternity Suit, then heterosexual men must be able to as well.
The scrapping or (at the very least) the re-definition of the concept of adultery. If adultery is understood as having coitus with someone other than your spouse, then this can obviously only apply to people in opposite-sex marriages [sic] and thus discriminates against people in same-sex "marriages". So, either the concept of adultery must be scrapped or re-defined to mean "engaging in any act of intimacy with someone other than your spouse". Given that the infinity of fetishes and the intensity of feeling that accompanies them, a married dacryphiliac could commit adultery simply by commiserating a friend at a funeral. And an act of intimacy does not necessarily aim at an orgasm. It could simply be spending time away from your spouse with friends, playing rummy. Therefore, my father (God rest his soul!) committed adultery with his rummy partners.
The end of sexual segregated bathrooms and lockerrooms. If heterosexuals feel awkward doing very private stuff in the presence of people who are not their intimate partners, homosexuals do as well. Yet, heterosexual awkwardness is considered whereas its homosexual counterpart is not. Conclusion: Sexually segregated bathrooms and lockerrooms are invidiously discriminatory and as such are wholly repugnant to Public Reason.
I'll think of more later. Feel free to add to the list.