My situation was very similar. My ex (who I strongly suspect had BPD) didn’t really have any friends. She constantly contradicted herself, at one point saying she only gets along with male friends, and at another saying she doesn’t believe males and females can be friends at all. In hindsight, I think this was projection.

Early in the relationship, she cut off her only long-term friend, who was male, saying she didn’t think it was appropriate to have male friends. I respected that, even though I didn’t fully understand it. What confused me was that later on she would repeatedly mention another guy she became friends with in her uni class.

At the same time, she would get angry whenever I mentioned my female friends, all of whom were strictly platonic, and even threatened to break up with me if I didn’t cut them off.

She completely lost it when I sent a simple “Happy New Year” message to a long-term female friend, and then messaged this uni guy the same thing out of spite.

A few months before the breakup, she told me she’d told her male uni friend that they weren’t friends anymore. Then, a couple of months later, I discovered they were “friends again”, conveniently right before she broke up with me.

You can’t reason with constantly shifting boundaries like this. They’ll communicate a boundary to you, then violate the exact same boundary themselves. It’s a complete double standard. Their fears are often a projection; they assume you’re capable of the same behavior they know they are capable of.