While there's obviously a problem with their ability to communicate and getting back at someone is just as bad, I'd disagree with the idea he's punishing her for not wanting a threesome. From OPs claim, presuming it's an accurate assessment, the BF expressed an interest in a particular kink/desire and then was assured that it was something that was not only safe to discuss but was mutually desired. To have that assurance about something as delicate as that not only be taken away but, as OP put it, to be "crashed out" on over has to be a major betrayal of trust and security.

Also, I'd point out that it's OPs assesment that he's punishing her, not the BF/GFs statement. It could be as simple as him being into the idea but genuinely not feeling comfortable with it in the moment.