Quote from Rearden Metal:
So what does a clever guy do instead? Never pledge monogamy, no matter what! (You can still get married though, by redefining what the word 'marriage' means to you and to her.) The basic philosophy goes like this, and it works both ways in the main committed relationship: "If you have to fuck someone, go for it... just don't lie to me!" This works much better than the normal way of doing things- it really does.
In case you're wondering how to pull this off: The key is to bring up this philosophy up-front, within the first week or two, when your relationship with her is just starting to get serious and nailing other chicks is the farthest thing from your mind. It helps a lot to pick someone who grew up in the 'old country'. Eastern European women aren't just hotter than the domestic variety, they also tend to be more tolerant.