There's a difference between an action and a belief. What if he didn't dis-associate himself and simply believed they shouldn't marry people of the same sex? That's not an action, it's simply a belief. Suppose he treated them well otherwise? Would dis-association with him be bigotry then? I would argue that it's at least cruel.
It did the opposite for me. When I was in highschool, I actually argued with a social teacher about "why should we care?" About gay people getting married. After going through college, my belief that gay people should be able to get married was solidified, but the idea that the government should be in the business of recognizing marriage at all was brought to the fore. Partially because people I knew were excessively rabid against people who were against gay marriage, and partially because I began to think about "open marriages" and polygamous relationships as viable forms of a sustainable relationship.
Marriage has been a social institution since way before any of the Abrahamic religions came into being. Sometimes it's used for love, sometimes for political unification, or as wergild, as a peace treaty - but it's always had a set of rights and privileges associated with it.
as a civil institution, marriage Is basically a bundle of various legal contracts - your spouse automatically (unless otherwise stated) becomes the primary beneficiary of your estate upon death. They are granted visitation rights upon hospitalization. You are able to jointly file taxes without needing to form a holding company. They are granted power of attorney.
these, and hundreds of other, legal arrangements can be made without marriage - they just each involve their own forms, arrangements, and processes. A Marriage contract is a large package of separate legal arrangements that have all conveniently been bundled together into one unit.
These arrangements are binary in nature, though - You can't have polygamous marriages because then you have multiple primary beneficiaries, multiple people with powers of attorney, and so on - How do you resolve a legal dispute where two people have power of attorney and both want to take different actions? You can't, without the courts throwing out the whole concept of power of attorney and taking it upon themselves to make the decision for the multiple individuals who were previously granted power of attorney.
An "Open Marriage" is simply a marriage where the two individuals agree to be sexually (and perhaps emotionally) involved with other individuals - but that doesn't change the legal arrangements they've made together. They've still agreed to give each other power of attorney, and so on.
