We all know that the marriage between Bolli and Nivenor is...unstable, at best. They're at odds over what they want, and it's implied that they're both cheating on each other.
So imagine my surprise when a thief wanders into Riften one night and the two of them immediately grab daggers and kill him together. At which point it occurred to me--this is exactly what the two of them need!
So, let's have some adrenaline sex between these guys. Maybe they've just taken down a thief or a vampire, maybe a dragon has even swept down on Riften and they were part of the fight that brought it down. Either way, all that fighting and teamwork reminds them of some pent-up emotion, and they proceed to have loud and energetic sex.