I don't know. I think it is possible. Straight couples are just as afflicted as we are.
One theory of mine is, homosexuals don't take monogamous relationships seriously because, still, a good part of the world doesn't take gay relationships seriously. If we're not expected to measure up to something, we're not as likely to.
I think since a homosexual relationship (a monogamous one) still isn't concretely defined in society because of lack of legal marriage (like a heterosexual monogamous marriage is) that we feel more open to BE open in our relationships. Being in a gay open relationship doesn't look too bad because there's really no more "honorable" type of a relationship to be in. For a straight couple to be in an open relationship, it looks sort of... trashy. It's expected of us, or considered normal... we have little better we can attempt to strive towards.
Marriage holds people together better, it makes you work through the trials. Do you need it to ensure faithfulness? Nah, but I think it can help.
Do I believe in monogamy? Sure. Am I worried? Yeah, I don't hold out a lot of hope for truly monogamous guys, I suppose I've been messed with too much.
Before I ceased repressing myself, I dated 4 girls, each one, unequivocally faithful; they were popular, and could've had any guy they wanted, but they stayed with me. I don't think I could get the same luck with 4 random guys.
Monogamy is possible only as long as there are two people, in love, who want it.
Just how rare are we? I pose that