Not married anymore after occult

I don't know if it's a Bug or a Feature, but i was playing with two Sims in my household. A male Vampire and a normal female Sim.

They got married and a few days later I wanted to turn my female Sim into a Fairy. So I went with bothe Sims to Innesgreen. After the fairy transformation the Game said that they're just lovebirds and not married. I checkt both of the Sims, but they were just a cuple not married.