I just want to say that I totally feel your pain.
I am a (mostly) hetero male, and I've been poly for about 3 years. I've been in a really solid, loving relationship for two years now. In that time, my partner has gone on countless dates and racked up about 15 sexual partners. I've had two dates and one sexual encounter, and been rejected more times than I care to remember. That's a repeating pattern from previous poly relationships, as well, and I've seen it in every other poly partnership I'm aware of. It's brutal on our egos.
However, as someone mentioned, that pattern holds true for monogamous folks as well. When I was mono, I'd break up with someone and then end up lonely and involuntarily celibate while my ex-partner slept with the local baseball team. And it sucked just as bad. Actually, it was a lot worse, because I didn't have the tools then to deal with my jealousy.
The quality / quantity issue definitely holds true. Despite her active dating life, my partner has been no more successful than I have at finding other partners. She's probably having more fun than I am, but that's just one of the rare perks of being a woman.
I don't know what to tell you except masturbate a lot, and cultivate close friendships and hobbies. And you're not alone.