"This is <sim name>'s bed"??

Whenever my sim(s) are single to start with the bed belongs to my sim (no popup saying This is <sim name>'s bed when hovering over sleep).... when they get into a relationship the bed is automatically flagged as the other-half's bed... why?
just want to know this logic of why the game assuming the bed isnt my sims, but my sims spouses (or other-half)

Edit: Correction "This is <sim name>'s bed"