Well, nicothoe, the need to keep sexual activities private differs from person to person and couple to couple. We have a huge thread here about that, with lots of different perspectives from people, so I think that is something Jen and Beo need to address according to their own standards.
Jen, there is also a big gigantic thread here about jealousy. Might be useful to read through it. But it actually sounds more like envy to me -- he wants what he perceives as something someone else is getting from you.
I know that Beo has struggled with this before. It seems to come and go in waves and I'm sure he's been in a more secure place about it, but now he's feeling not so secure. Is there anything that prompted this? We have not heard from you in a while, so what's been going on? Have you two continued therapy? And what about the desires for someone else that he had discovered last time he posted here? Has that changed?
It's always helpful to look at jealousy (or envy) beyond its face value and deconstruct it to see what's underneath. What is the dialogue Beo has with himself when he feels this way? If he feels like he's only seeing you for the mundane, everyday things that take place in a marriage, maybe you need to romance him some more, go on dates, etc. But he has to really examine what's under the surface of his jealousy and uncomfortable feelings. It could very well be an inside job, a self-torturous pattern of thinking that only he can get himself out of. In that case, all you can do is love and reassure him as he works it out himself.