My husband of a little over year with newly diagnosed BPD has a habit of threatening me that he will find someone else when he is angry at me especially when I need to leave because of the volatility of the arguement. Its hard to know how to feel about it (besides hurt) because I dont know if he will act on it or already has. Even though he has been married twice before, he said he was always faithful and slept with only one other person after them and before me. He was sexually abused as a boy and has some sexual issues like inhibition and says he would never cheat because he knows how it feels to be betrayed. Its his go to phrase to hurt me and I understand the tendencies of some with BPD to "fill the need" but also know he has sexual issues that would keep him from doing that. He will be starting therapy soon but dont know how to handle his threats when he's hurt or angry.
That's really, really hard to deal with. I've had to deal with shades of infidelity with my SO from the beginning. It is characteristic of the nature of the disorder to behave impulsively, which sometimes involves sex.
In order to cope with that constant threat of infidelity (which is more about betraying my trust than it is sex), I just try to center myself on the thought that if/when I do find out about her being fully unfaithful, that will be my clear and indisputable out. In the meantime, it does me no good to focus on something that has probably happened (given the circumstantial evidence) but I have no irrefutable evidence to show it.
In other words, I will deal with it when that day comes.