It's quite common. My vet says it may be because my Mishi was neutered after having had "experience." He has his "girlfriend," Muffy the teddy bear that he humps regularly. I just laugh, sometimes take pictures for PT (see the Purrno thread) and call him my goofy guy. I often ask if he needs to be be left alone, my silly, amorous, oh-so-horny boy. There are other kitties here on PT who do the same thing, so you're not alone.