I have a similar issue. The thought of being limited to only 1 girl for X years or forever frustrates me, but I don't feel right when I cheat. No matter how strong my resolve of "I will be single forever" after a LTR ends I always end up catching one-itis again down the road.
Open relationship doesn't work for me, the frustration of not being able to play doesn't feel worse than the feeling of her being with other guys.
What works for me is to only get serious with bi-sexual girls that are genuinely interested in sarging girls with me for
threesomes. I say "genuinely" because I have found out the hard way that when a girl does it only because she's scared of losing you if she doesn't then it just makes things worse.
Know what you want and qualify early, I bring the subject up from day 1 and don't bother with girls that don't seem excited by the idea. I will at some point, after plenty of fantasizing with her, tell her "I want you to do this because it's something that you want to do, you don't have to do this to get me to like you more, if it never happens that's totally cool" and leave the next initiative with her after that.
Anyway, that's how I compromised with my nature
