The central conflict in those narratives often isn't about physical desire but emotional possession, which cuts way deeper. That feeling of being replaced on a soul-deep level, of watching someone you trust rewrite their entire world around someone new—it’s a specific kind of devastation. The tension comes from the slow, painful realization, not a sudden reveal. The reader gets to sit in that dread, feeling every glance, every missed call, every little emotional withdrawal.
A story that really crystallized this for me wasn’t even a book, it was a visual novel called 'Kuro to Kin no Akanai Kagi.' The protagonist's gradual understanding that his partner’s submission was being willingly given elsewhere, that her deepest vulnerabilities were being shared with another, was brutal. It wasn't the sex scenes that hurt; it was the quiet moments after, where you saw the emotional landscape permanently altered. That’s the grip: it forces you to witness the dismantling of one reality and the construction of another, and you’re powerless to stop it.
Ultimately, it plays on a fundamental fear of being not just left, but deemed insufficient on a level that matters most. The ‘gripping’ part is the morbid curiosity of how far that wound can go.