Welcome! I think we all struggle for a while with the go/stay dilemma.
Expecting him to change is probably futile. However, YOU can change, and that can have positive effects on your sanity, and sometimes the relationship.
Trying to talk through our conflicts doesn't seem to work.
Talking with a BPD is different to talking with a non. "Problem Solving" is often impossible. Instead, conversations should focus on how they FEEL, just to let them vent. That often makes them feel better straight away. A good conversation technique is Validation.
[Female friends] When I try to explain that this isn't normal behavior in a marriage... .
Don't try to change him. It won't work. Instead, do one of two things:
1) Realise that this is your own insecurities talking (which is natural). You worry that he will have an affair or something similar. You may need to work on this yourself.
2) Impose a "boundary". A boundary is something to protect yourself, and normally in the form
"I don't like X. When you do X, I will protect myself my doing Y". It is NOT aimed to change his behaviour. For him socialising with women, I can't think of an appropriate boundary. But
"I don't condone affairs. If you have an affair, I will leave you" would work (but only if you *DO* leave him - this is not supposed to be a threat).
Read about Validation and Boundaries here:
https://bpdfamily.com/message_board/index.php?board=36.0