The thing about marriage...

In my first playthrough I married Sylgja. Her quest to bring her satchel to Dark Water Crossing to her father, Verner Rock-Chucker. Now I just went through with it, married her, no problem.

Some time later, I was for some reason back in Dark Water Crossing, and I managed to finish Annekke Crag-Jumper's quest and got her as a follower. She stayed with me for quite some time before being forced to leave during one of the Companion quests.

Fast forward to yesterday, when I was playing as my second character. I was at Dark Water Crossing again, and this time when I was about to deliver the same satchel, I found the two of them in bed together.

That was when I realised that they were married.

And I was traveling with my mother-in-law for a solid 10 hours.