For me it depends so much on the circumstances.
Some gay couples allow it if sex with the third person occurs only as a threesome, so both are involved together.
Some are the opposite, allow it only if it is never talked about.
Others, only if it is talked about and all the rules are ironed out.
I've seen all of these situations in my friends.
Myself, I am open to any arrangement above.
I have read a poll on open realationships in a book, dont ask me which one, I picked it off a library shelf years ago. I think it was of America, I dont remember. It said that something like 90% or more of gay male couples had open relationships after a few years. They start closed, then when the romance dies down, they become open, and the relationships are just as stable as closed ones.
Personally, I feel open relationships can, in many cases, be far more stable than closed ones--it depends on the individuals involved. Some can do it, some cant, some need it, some dont.
It also discussed lesbians. I dont remember the figure that preferred open relationships, but it was a majority--really surprised me.
With all I've seen and heard from all my gay friends over the past decades, I have absolutely no problem believing the results of this poll, at least for a big very gay and very liberal city--where I live, maybe not for heartland America. I am a bit surprised by so many negative reactions to open relationships here.