This question was actually posed a while back but it was kinda ignored and I'd like to bring the thread back but I cant seem to find it.
If a man found himself in the position where he was interested, romantically, with two women, what would be the issue (if any) of him bringing the two women into a relationship at the same time (Both willingly).
I'm pleased to say that I have been one-third of that situation in the past. Although it didn't work out for all of us in the end, mainly because of time constraints and practical concerns, it was an experience that I hold very dear. In our specific situation, we engaged in a perfect triangle where each of the three of us was romantically involved with both of the other two. There were also times when all three of us would spend some "quality time" together
Currently I am in a relationship and my boyfriend is "allowed" to have sexual relations with other women, with two constraints: I must know about it, preferably in advance, and he must protect our sexual health. (By saying that he is "allowed", I don't mean that I would prohibit him from doing anything at all, because that is not my right, but we both feel we have the right to make it known to the other what would hurt or upset us.) I'm quite comfortable with this situation. I don't feel any jealousy or anger, and in all honesty I often find him even more affectionate and loving when he has been involved with someone else.
Theoretically I see no moral problems with romantic or sexual affection being shared between multiple partners. Personally I have been blessed with a freedom from sexual jealousy. It doesn't upset me to know that my lover is having sex with someone else, or even to see that happening. However, the situation clearly can't work for everyone - all parties must consent, and there must be a mechanism for dealing with any grievances that any party might have. I'd also like to add that you only mention a man in your example, but I don't see any reason why a woman shouldn't have more than one male or female partner if everyone involved is okay with that.
Ultimately I think the ability to rise above jealousy and possessiveness is a valuable one, and I don't really understand why jealousy in the case of sexual infidelity is considered so virtuous and righteous. When I love someone, I want them to have things that make them happy. Sex is fun, and I would never deny my lover safe, consensual sex if that's what (s)he wants.