This conversation has touched on a subject I feel strongly about.
While I don't feel people should (have to) be limited to monogamous relationships, I detest dishonesty. One thing I am adamant about, when in a relationship with someone, is being honest and communicating your feelings and intentions with each other. (Although, some discretion is advised as it does no good to discuss the details of your sexual affairs with your partner, unless of course that's something that turns you both on.) If monogamy isn't for
you ("you" being used in the proverbial sense to address everyone), I urge you find someone like-minded and have an open relationship, rather than lying and cheating and eventually breaking someone's heart. This is for your own benefit as well as your partner's, as I'm sure you'll also be happier not having to skulk about behind their back.
Needless to say, I think the world would be a much better place if people (being the mature adults they should be) were just honest and decent towards one another.
It would suck being either one of these guys (or even the girl, as I've implied above) in this situation.
That's not to say there aren't people that get a thrill from the act of cheating on someone, but fuck those people. (No pun intended.)