Why is girls being Bisexual "hot"Watch
But it also could not be that deep. Idk your bf so I don’t want to be making assumptions about him. But maybe ask him why he views it as hot especially if it made you uncomfortable.
Also, they might watch lesbian porn and enjoy girl on girl um... action (so they imagine you with another girl).
I think those are the reasons.
Maybe he didn't know how to appropriately respond.