Not so much jealousy as feeling like a loser
In my first officially non-monogamous relationship, we both wanted it from the first date (9 months ago) and we're a pretty awesome couple. She is constantly telling me how great I am and how she can't imagine ever leaving me, she doesn't understand why I'm not constantly surrounded by women, etc. I trust her and am not worried about her leaving me.
Since we started dating, I've tried going out with 3 other girls, weren't really into each other, pretty awkward, nothing happened. I'm really shy and maybe a little picky, so that's pretty typical for me?
Meanwhile, I can't even keep track of all the guys she's gone out with. I can think of 8, but there are probably others I'm forgetting (several with the same names). Went on several dates each, excited about them, made out with most of them, had sex with at least 4. It's kind of intimidating.
Likewise, the latest guy she's seeing already has a wife and a girlfriend, and after their date tonight I find out that he's also currently dating another girl, and is interested in yet another... who just happens to be one of the three I unsuccessfully went out with. :/
Before we met, I hadn't kissed anyone in over a year, and I was trapped in self-reinforcing feelings of insecurity, loneliness, and depression. Now that I'm seeing just how easy dating is for everyone else, how they're all having such a great time, and I'm just seeing her and struggling elsewhere, I'm beginning to feel the same as I did then. It's bizarre to feel lonely or insecure when I'm in such a happy, stable relationship, but I can't help but compare myself to the success of others and feel like a loser, or feel that I'm somehow being taken advantage of, though I know that's not the case.