I'm not sure how common this is but our male cat, Sencha, likes to hump his favorite stuff animal (a dog) whenever both my husband and I are in bed. He is neutered.
Our bedroom is on the 2nd floor and he likes to get it on on the first floor. He will only "have sex" with the dog when he believe both of us are in bed (it doesn't matter if we are awake or not). Also when he has sex, he meows very loudly.
Tonight before I got in bed I noticed his dog was right besides our bed. I then got into bed. Within a minute, I heard little footsteps in our bedroom followed by noises near the staircase and then loud meowing downstairs. Due to a pet camera I was able to capture Sencha grabbing his dog before heading downstairs.
Here are some other pictures of Sencha with his dog.
My neutered male cat will hump a specific blanket if I leave it out, so I think fairly common.