I had a monogamous-in-fact relationship for 36 years—literally until death because the nice fellow died unexpectedly three years ago. I'm saying monogamous-in-fact because we started out with the theory that we weren't necessarily going to be monogamous, that we'd work it out when the time came, we would be honest, etc. But after a couple of non-events we said, "Well, about that... it seems like more work and stress than we actually want to go through, huh?" Then we adjusted our general plan to be "monogamous unless we have a change of heart and then we'll see," but pretty quickly we realized we were too lazy to deal with the complications of non-monogamy.
It worked out well for us. Like I said, we were lazy people. My guy really liked visual pornography, and I like prose, so it's not like we turned our erotic brains off. I think that monogamy was an easy, comfortable lifestyle for us, rather than as a constraint, which is why it felt like a resonable choice for us. We each had some strong attractions to other people over the years, and I had a feeling for somebody once strongly enough to talk about it a lot for a while, but when it came down to it, I really really liked my nice fellow and I really, really liked what we did together, and occasionally thinking about other men was enough variety for me.
In some ways I think that what we had was more like the "monogamish" relationships you describe, Dan, except for the fact that we never actually did end up having sex with anybody else.
An anecdote that seems relevant to me: one of the bars in the area started having weekly "lingerie shows" which meant women walking around in their underwear. When the nice fellow told me he was going to go with a friend, the friend was aghast: "Why did you tell her!" I laughed at him. Apparently the friend thought there was something sexist and shameful about looking at women in pretty underwear, and he was terribly embarrassed that I knew they were going. When I explained that, in fact, it was possible for a man to be genuinely interested in women's bodies, to enjoy sexy things without being sexist, it was like he had never heard anything like that before. I wouldn't have wanted to be with a man who thought that being "faithful" meant grimly denying himself any pleasures beyond our bed. For one thing, he might have expected it of me and then how could I justify reading so much gay porn?
Relationship Open Without Fucking Other People