This is when someone has a thing for dead bodies, namely necrophilia. Do you really need the details?
May result from or in Drop Dead Gorgeous, exposed corpses.
Note that while Mummies at the Dinner Table may or may not involve necrophiliac behavior, in Mummies at the Dinner Table the character is attracted to a corpse because it was their beloved while alive and they just can't let go. In this trope, the character is attracted to a corpse because it is a corpse.
Probably the only way to make this even worse is when post-mortem dissection is involved.
If someone has a relationship with a member of The Undead, it's more a case of Boy Meets Ghoul. Compare: Mummies at the Dinner Table, Necromantic.