So is it normal for a pwBPD to talk about sex with someone outside the relationship in such great detail?
If that's what she felt the girlfriend wanted, yes. Borderlines evaluate people's needs (sometimes incorrectly) and then follow that perception of need as a reward. If you confront them, you will get the "I thought that's what she wanted!" statements. In effect, offering themselves up to be valued based upon a perception of need coming from another human being.
Gossip can be very rewarding to a Borderline if they can provide it based upon the perception of need to another human being. They have no idea of reference where they begin and others drop off, so they attempt to merge with an object to seek an attachment by providing information that appears to value them as objects.
It doesn't matter if the information is slander, hearsay or even fantastic made up stories, the Borderline regards the "want" of the listener as an opportunity to merge with them and have a bonding experience. This has been taught from childhood and due to nonexistent boundaries of where the self to "other" begins and ends.
Borderlines will take information given at close range and readily share it with others if they find reward in it. Since there is no awareness of boundaries, there are no expectations of boundaries, there is only an act of sharing, and goal to acquire reward. Do they feel badly about it later? Yes, sometimes they feel very badly, but this is a self fulfilling prophecy of defectiveness and the disorder moves unconsciously until it is confronted. When confronted, it becomes criticism, which they expect. So they unconsciously move towards reward in fantasy merge and then withdraw from criticism of that oversharing in reality.
Borderline is a persecution complex- and expectations of persecution are already in the psyche. In a way, it is a built-in self defeat of personal boundary setting.
