Boring sex can take many forms and have many causes in my opinion. But that is something else.
The most boring sex I have ever had was in Vegas some years ago. The guy came to my room we got naked and all dont' think there was much if any foreplay. He wanted me on my stomach which was fine, he got on top pushed his cock in and then just laid there. He barely every moved he may have twiched a couple times but didn't fuck or anything, this went on for a good 20 minutes or so he eventual shot his load got up dressed and left. The entire time I was thinking about my meeting the next day and what I needed to do when I got home.