Problem solved! In a weird way though...
Moving them to another house didn't work. And I really didn't want to move them to another town so I thought of something else.
Since I had them getting married at one point (you know, with the fish on wood in front of them), I went back to that save. Little did I know about cheats like "Testingcheatsenabled true", but I tried it and with pressing shift I got the option "remove object" and so I did
That was a save in the middle of them getting married. They were actually waiting under the wedding arch at the time. I could choose the option "marry" Yey!
The weirdest thing was that after they were married I got a bunch of messages in a row on the top right of my screen saying things like:
"she became friends with him"
"she became lovers with him"
"she became fiancee with him"
I don't know the exact right terms, but you'll understand what I mean. I got the same messages for him.
They were actually fiancees, but it was as if the game saw them as 2 sims without a relationship and after this wedding they got all states of relationships at once.
Ain't that weird?
Hopefully my glitch is gone now!