A final update:
I've completely lost interest in C. He had said we should hang out as friends with the possibility of something romantic. Then about a month later, he mentioned something that made me ask, "Wait, aren't you and K open?" He said, "No, no! We're monogamous." I said, "How long have you been monogamous?" He said, "About two weeks, I guess."
I said I would have liked to have known since I'd still harbored romantic feelings (especially since he lives 50 feet away from me and we see each other daily!).
He explained that they'd become monogamous again because "She's eating healthy, exercising, taking care of herself, and she's more stable now."
So basically, they were open because he was unsatisfied in the relationship and was hedging his bets! Honestly, I find that disgusting, and he was inconsiderate not to tell me they were no longer open.
So, basically - we're back to acquaintances now. And I'm probably much better off!
Me: 22-year-old female, cis and queer, have identified as poly for ~2 years, currently in my first committed poly relationship
A: Poly boyfriend since 9/17/13, currently sexually open and not seeing other romantic partners but open to such in the future