In this circumstance, I think you can also separate different behaviours and desires.
In the context of a romantic relationship, this other guy only wants monogamy. That doesn't mean a romantic relationship is the only form of relationship he wants. Why should it bother him that his fuck buddy has a boyfriend?
I would never buy a home that wasn't single-detached house with a nice yard that could fit a garden. But that doesn't mean I would only live in a single-detached house. When I've rented, I've taken what I could get and what I could afford. I would never buy a condo, but I would rent a suite in an apartment complex.
So, the fact that he's still fucking your girlfriend when he doesn't want to be her secondary boyfriend doesn't mean he's trying to steal her away to be his monogamous girlfriend. It means he likes to put his penis in her vagina, nothing more.
Honestly, I would be far more concerned about why your girlfriend is so unstable that she's sinking into inconsolable depression just because some casual dating partner won't be her new boyfriend.
Last edited by SchrodingersCat; 06-22-2013 at 02:58 AM.