This has been a really interesting thread, I hadn't seen it before so I'm glad it was resurrected.
Something that hasn't been mentioned but has always occured to me as a theory about gay open relationships is that it could be about numbers as much as anything. Plenty of straight people struggle to find lasting love in a world where they have 95% of the population to choose from. Is it then surprising that gay people struggle when they only have 5%? I could well imagine that this would lead to a scenario where someone finds a companion who they love, but possibly aren't that attracted to or are sexually compatible with, and therefore decide to compromise on the relationship.
I can follow the logic of the arguments being made, but I do think they start from a rather idealised version of marriage. A quick peruse of Mumsnet will show how many men cheat, and how many women do so as well. That's before you get on to the women who happily carry on relationships with men they know to be married with children, and in many cases actively campaign for those marriages to end. I've known several in RL, so I wouldn't say that it's rare.
Promiscuity is undoubtedly higher amongst gay men, but there are plenty of straight swingers out there, just look at the leaks of data from Ashley Madison and Adult Friend Finder. Again I've known people in real life, male and female, who swing, so probably not as uncommon as people think (I should add here that I've only been married to one man, faithfully, so I don't know these people because I go out of my way to meet them).
The fact that 30-50% of gay men have open relationships may shock, but wherever divorce is allowed freely I believe there are similar rates of divorce, which given the numbers game, will mostly be amongst straight people. Is serial monogamy better than an open relationship?
The point I'm making is that if I accept marriage is an ideal to aspire to, and I do, given that we straights so often fail to live up to it, I don't see why gay men shouldn't be given the opportunity to fail at it too, certainly when it comes to civic marriage in a secular country. That's been my conclusion anyway.
I would say that I'm in the UK and a Protestant, which you may feel explains my differing conclusion.