Time time time.
Seriously, I think I crush out on about every third person I meet, but it usually dies out after a few interactions...and then new friends! In my relationship, flirting & flings are no big deal (as opposed to actually seeing someone consistently, which requires a bit of talking and consideration), so I could technically act on anything I want to. That doesn't mean it's the best idea, anyway. Beyond your and your partners' needs, you've got to consider your crushes' needs/wants too. Even if you're in a really very open situation, you're still going to deal with not being able to (considerately) act on some (or much) of what you might fantasize about. It's a messy business all around.
If the guidelines of your relationship are constricting you, that's something to figure out and talk about. But beyond that, my biggest advice is to just try and enjoy the feeling of crushing/lusting. It's fun. Fantasies are fun. And sharing those fantasies with a partner (without necessarily needing to act on them) can be a nice way to connect and deal with jealousy and insecurity.