It wasn't that long ago that I was a kid in high school, and I used to get very jealous when I saw my boyfriend flirting with someone else. Especially after Boyfriend #2 crushed my heart. I learned to relax a bit more and let things take their course after breaking up with Boyfriend #3. He was a true monster in every regard, a total control-freak. I resolved to never let myself become like him. As a result, I don't normally get jealous with my husband. He sometimes goes out after work to have a drink or two with his coworkers, but always comes home in time for dinner. He's allowed to have fun, of course. Everyone is. But I get a little protective if we're at a party and I think someone is moving in on my guy. Hands off! Mine!
I guess the rules change a bit when you're married. A little bit of jealousy is okay, just to remind people that your partner is already taken, but you don't have to worry too much. After all, he/she said yes to you, not the other guy.