Well here are the reasons why you should remain monogamous, but it's really up to you:
New experiences aren't always so good
Most of the time, it is simply the promise of a new sexual encounter, of discovering someone at their most vulnerable, that excites, not the encounter itself. Short-lived, half-drunk fumbling and the awkward time in between waking up and leaving? No thanks.
You're not with the real person for a long time
How long does it take to truly relax around another human being? Ages. So when you fool around with someone, you are more than likely fooling around with your own image of that person, or at least, the image they want you to have of them. And you aren't really being yourself either.
What needs can be met by a one night stand?
At some point one might argue that stepping outside of a relationship might fulfill some need. But what need can actually be met by having random sexual encounters (except the need to have random sexual encounters?). If you were in a relationship with a person that cared about you, that person would care about meeting your needs, so perhaps the first thing to do would be to talk openly and honestly and try and find a way for that to happen.
Fun and excitement! Always!
Again those against monogamy might argue, things get boring if you're with the same person all the time. We humans need fun and excitement in our lives! Sorry to be a killjoy, but if you constantly need ‘fun and excitement' in your life, you're not really a grown-up. ‘Fun and excitement' should be replaced by loving kindness and a fulfilling connection that deepens the sexual experience. Real adults experiment with tantra. Those that think they're adults because they have credit cards go to bars to get laid.....