I am not a jealous person. Nor have I ever been with a jealous person. I have seen jealousy ruin otherwise good relationships of friends and former roommates before though.
One excuse I've heard several times to justify jealous is: It's not that I don't trust him/her; I just don't trust other men/women. Oh yes, on the surface it may seem that you're being concerned and protective using this line of reasoning, but what you're really saying is that you don't trust you partner to handle the situation in an appropriate way. You're using your insecurity to rationalize controlling your partner.
My college roommate had a really jealous boyfriend. They had been together since high school but he went to a different college. He would drive down to our school every weekend to be with her. On the weekends he couldn't be with her, he told her he didn't want her going out because he didn't trust other guys. He didn't even want her going out with me. On the rare occasions she would go out anyway, he would page her (this was before cell phones) every fifteen minutes to "check up" on her. She almost got kicked out of the movies once because she wouldn't turn off her pager, because if she did, he'd get mad. After a year, he gave up a diving scholarship and transferred to our school so he could "look out" for her. You can probably guess how this story ends. She dumped him and eventually married one of his friends.
On the other end of the spectrum: When Chuk and I had been dating for about six months he told me that two of his friends were going to be staying with him for "awhile" until they could find an apartment. By staying with him, he meant they'd be sharing his room. It just so happened that both of these friends were women, and one was his ex-girlfriend. Obviously, I wasn't too happy with this situation, but he didn't ask me for my opinion, he just told me that that was what was happening.
I thought about objecting, as I think most people would in my situation, but ultimately I decided against it. Chuk had never done anything to cause me to distrust him or question his devotion to me. These women, even though I had never met them, had also never done anything for me to question their trustworthiness. I also figured that he and his ex broke up for a reason. Had they wanted to get back together, they could have before Chuk and I met. They didn't.
What was the result of me choosing to turn away from jealousy and towards trust? We are all great friends now. We still talk about how much fun that summer was when we all hung out and had barbeques all the time.
Here's my theory on cheating: If your partner wants to cheat on you, he/she will. There's nothing you can do to stop it, so jealousy is just wasted energy. Instead, use that energy to build a stronger relationship. If your partner cheats, you can walk away knowing your energies were better spent.
Jealousy is easy. (Aren't all fear-based emotions easy?) Trust is hard, particularly if you've been hurt before, but from trust comes respect, the cornerstone of a healthy relationship.