Trying to answer the original question--
As others have said, this is a tough one. I think it truly depends on the situation/culture/society...a combination of your personal issue and what others in your environment as well as community think.
In my honest opinion, it can only work if EVERYONE is honest. I am even in this sort of scenario myself to a point...since I'm engaged to a man who wants marriage and family life but also wants one-night stands with others. The difference is not only that he is completely straight-forward and honest with me about it, but he also really believes that it is to fulfill the needs of these prospective 'other women' too, emotional and/or physical.
It took me a long time to understand, for at first I simply thought he was joking and then had a long period of jealous bouts because I thought his reason was purely sexual and selfish. (The stereotypical "that guy is a pig" mentality) I still feel jealous and bitter part of the time, but I've noticed it is literally diminishing for good, and I may be able to accept it.
I still question at times if my guy is confusing his actual needs/desires with fantasies. It seems like he wants everything he wishes for to be a reality--something I find to be positive, but to a point totally unrealistic. He says no other long-term relationship he's had has fulfilled all of his "needs" and he puts a lot of "esteem" in me as a result. He says if it didn't work with me he would rather pursue his interests without a partner instead.
This isn't really an ultimatum, again he's just being totally honest. It's strange, and I find it highly unusual for men to be this way...it also goes against much of what I learned/assumed growing up. But since I am flexible and open-minded concerning this I decided to give the whole situation a chance.
My point is as a person who will soon be a wife to a guy who very likely will be with other women (possibly sexually, possibly not depending on who they are) and who wishes to stay totally honest with me and with them, and even include me in it if I wish...I can't see anything inherently wrong with it unless anyone disagrees and it goes against their individual beliefs/morals. So really, if the man is straight-forward and honest, it's up to you. And if they're lying and trying to hide it, it's THEIR problem and I would say get the hell out. We all deserve the best, and if you can't live with who they are or what they do, then they don't deserve you.
I hope this is a help...best wishes to everyone!