Where was Bee regarding Jay's decision to start seeing another person while she was pregnant, I presume with their child? Was this mutually agreed upon, or did Jay make the decision for her?
Did he make it with her active input, or did he make it knowing "she's a hermit"? If he uses that label to explain away her retreat into her room, what does that say about the amount of work he is willing to do to seek her active input? (Particularly if as AnneITR notes, Bee is one of the folks who goes quiet when her needs aren't met, or doesn't know how to ask for them.)
Had they, and you, sat down and realistically discussed the very real limits on time and attention that all humans have, and people in late pregnancy/child infancy have in spades?
There are times when getting a fair share and and equal share of attention are two very different things. And let's not forget there is now another person in the mix--not just you and Jay, and Bee, but now a new small human, very much in need of intensive focused adult attention to get its emotional, social, and physical life off on a good foot.
I'm guessing you're going to have to figure out what you consider a fair share of attention, and what you consider an equal one. Are you saying that Jay should be giving you the same amount of time/attention as Bee? Are you saying that Bee should be giving you some share of time/attention? How much? Of what type? And who is advocating for the new little infant's needs? What is a fair and equal share for her/him? If Bee has difficulty communicating her needs as an adult, and the little one has only cries, can you see that your ability to articulate your needs gives you a form of power in this configuration? What is it that you really want?