There are numerous and complex reasons men stay in relationships with BPD's, and it's not at all uncommon.
Some reasons: Fear of retaliation, threats of false reports of domestic violence/rape/child abuse, fear of not being believed, hope that behavior will improve, fear of losing children/job/retirement/home, fear of being alone, buying into the BPD's contant declarations that the man is worthless/useless/ugly/undesirable/lazy/mean/etc, thinking no one else would want them, habit...and I could go on and on. Maybe most importantly, these men lose perspective on what's normal and what's abnormal because their boundaries and autonomy have been systematically destroyed. IMO