I think there's actually a split in the pro-LGBT community on the issue of marriage. There is a conservative wing of it that basically says that traditional marriage is just fine, it just needs to be extended to same-sex couples. This is typical of Christian supporters of SSM such as Matthew Vines. They believe in such things as lifetime commitment, two and only two partners, marital fidelity, etc., they just think that for people who are gay in their orientation, they should exercise that with a partner of the same sex. Such people would be embarrassed, I think by Richard Carrier's adaptation of LGBT rhetoric to support polyamory.
People like Gessen are on the radical wing of the pro-LGBT movement, they are happy to see the opposite sex requirement eliminated for marriage, but they reject the rest of traditional marriage as well.
The difference between these two wings has been de-emphasized during the battle for SSM. After Obergfell, I strongly suspect that this disagreement develop more and more into a debate within the pro-LGBT community.