SC, good point that the uneven setup could be causing him resentment. I mean, if my partner and I were both vegetarians and we decided to broaden our diet a little, and I liked both chicken and fish but he just liked fish, I wouldn't say "Ok, you can have all the chicken you want and I'll enjoy some nice fish! Mmmm, yum." and then expect him NOT to feel slighted.
What the heck is the point of these gender restrictions? I know that wasn't the question you were asking, but since it seems like it could be relevant, I can't help pointing it out. People are people, so what should it matter? What if you woke up tomorrow with a penis (or, slightly more realistically though I assume still a long shot, you decided to transition from female to male)... would she have to dump you?? And if another girlfriend is what would be really fulfilling to him, not a boyfriend, then how is he supposed to sit there watching the two of you snuggle and not feel jealous??
Anyways, I would be a good friend and partner to your gf and give her the best counsel you can about her relationship with the bf, including encouraging her to spend more time with him right now if that's what he seems to need. Talking to him about it would be much trickier. But ultimately it's up to them to resolve.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.