Faithfulness in Relationships - How possible?

Update: One of the comments raises an interesting angle. Should one partner's faithfulness be dependent on the actions/inactions of the other?

"Faithfulness isn't a trait you can sit back and expect your partner to just have. e.g. If I want to be the only one my partner has intimate conversations with, I need to let her know that. My role would be to always be available to her for that. If she goes off and has intimate conversations with someone else, i'd say she has been unfaithful. If I didnt make myself readily available, I can't totally blame her." - Think-about-it

We've read about how Dunni was broken-hearted by her former boyfriend's infidelity. In fact, she caught him with another woman almost pants down after they'd been dating for two years. In his defense, he said he wasn't the monogamous type of guy and asked her to understand. So the question is, how possible is it for people to remain faithful to one person in a long-term relationship?

I don't know if you've heard of the 2-year itch, there's also the 5-year and 7-year versions. This is a situation where people in a longterm relationship develop a desire to check out. Their eyes begin to wander, they find the grass greener on the other side, another man or woman becomes highly desirable, and in most cases, they cheat on their current partner or spouse. Especially on the part of women, many of us believe that men cannot be faithful, and that's the reason to forgive them if they stray.

Is this true? Can men not be monogamous? What of women?