I think that gay men are capable of maintaining stable, long-term relationships. I also think that it has been far more difficult for gay people to do so than it has been for heterosexuals.
In our puritanical USA, gay relationships have been relegated to shadows out of fear of losing a job, family, social status and anything else that would occur, if that person's surrounding people became aware of his or her homosexuality. That was NEVER the case with heterosexual relationships.
This has contributed greatly to ensuring that gay sexual encounters remain just that - encounters.
As gay relationships become mainstream, the idea that gay people cannot remain in long-term relationships should be revisited periodically. I think that as time passes, society will see more and more long-term gay relationships that it does now.