My guy friends always ask me questions like"why do girls only seem to want me when I'm in a relationship?" I have the same answer every time. Sorry, it is not a coincidence, or the fact that you are just really unlucky and the beautiful girls only flock to you when you're knee-deep in monogamy. One thing directly corresponds with the other. Fact.
For example, how many guys want to break up with their girlfriends, but would never be cool with her seeing another guy. Men and women, in general, are very competitive people who tend to have a lot of pride. It is human nature to only want something because you can't have it (which as I said before, most likely means you don't actually want it). If this is why you stay with someone- you both will be miserable, if you aren't already. No one ever wants to picture their ex/ current girlfriend with someone else, but you have to remember there is probably a significant reason that person isn't going to make it to your future.
At one point or another in everyones life, we have all wanted something, simply because we couldn't have it. Maybe it was a girl or guy, a job, an apartment, awesome jeans, etc. but it's happened to everyone whether you are consciously aware of it or not. It's human nature, we all like things or people we can keep at an arms distance. It is comforting. The fact that you can't have it makes the object or person seem "scarce", use supply and demand as an analogy. Here's my take on it- 99 percent of the time when you want something you can't have, you don't actually want it. And the other one percent, that is the exception to the rule. The other one percent falls under the Counting Crows 'Big Yellow Taxi' department.