I would love to know the actual number here, as someone in the gay male open relationship category.
I remember discussing this with a friend of mine, who is also in an open relationship. He came out with a fascinating statement that he had never seen a long term monogamous gay relationship where both parties were perfectly happy. After he said it, my first thought was that was ridiculous and then tried to think of friends who were the exception... I couldn't name a single one either. Every monogamous gay couple I've met going for longer than >1 year had at least one person in the relationship air that they were sexually frustrated to me in private and wanted more. Most were jealous of our relationship, but worried about the consequences of opening their relationship.
I am sure the monogamous gay couple who are happy is out there somewhere, but I've yet to meet them.