Our pwBPD will take small interactions with others as threats to the relationship. I've learned that when my H begins to make accusations, he is frequently feeling insecure about himself. Validation can go very far in these situations. "I can see how thinking I'm cheating would be very upsetting. I would also be hurt if I thought you were cheating." The more you try to excuse, cover up, or fix the accusations, the worse it will get.
I also have a no meeting privately with members of the opposite sex rule. Not that I plan to or want to cheat, but I never put myself into a situation where the temptation to cheat could happen. Personally, if my H set up meeting another woman without telling me, I'd be pretty upset too (and I am not a jealous person). I don't think it's a requirement for all couples, but it would be a smart rule.
Here is a link to one of our workshops on How to Deal with a Jealous Partner:
https://bpdfamily.com/message_board/index.php?topic=78324.0