Marg:
You are not alone in this. My husband and I got our current cat when he was about 3 months old. He was neutered around 10 months, once he was heavy enough. I didn't notice this masturbatory behavior until he was around 2 years old...
It started out with a fleece blanket that he loved. I always thought he was just doing the "pumper-feet" thing, but then it started to look like he was really digging in with his groin...
Then eventually he started exhibiting this behavior with me. Sometimes he just kneads, or pumper-feets me, but then he started to grind his lower body onto me too, not just the normal front feet kneading. For a while I was in denial that he could really be masturbating on me, since he has been neutered since he was a kitten, and has never had sex. He is also our only cat, so he is never around females, and has never had sex so I thought it was just impossible and I was crazy. But then one time I lifted him up to see if his penis was exposed, and to my shock and horror it was! I tossed him off me, pretty freaked out.
Since that time I realized for sure what he was doing, he does it rather frequently, but only when I am laying down, like if I am laying on the couch reading. But only with me, and never with my husband. However, like 95% of the time, it doesn't seem like his penis is exposed, which is odd (I check a lot since that time because it's really just too gross to think my cat is faux-raping me). I've tried to look into it, and there isn't a lot of information about it. I found this article (http://www.drjeffnichol.com/?p=485) that says he needs more activity or cat stimulation, i.e. playing, etc. and am getting ready to buy him a brother so he can have a friend to play with.
I really don't know what to do, and for now, I just let him do it, and usually he just stops on his own after a minute or two, or I move and he just leaves. Other people have said that their cats get aggressive when they make them stop (based on other posts I have read), and find another "source" of relief, but he doesn't do either of those things. If I make him stop, he just stops, and is totally normal. Like I said, I do not know what to do, and I am really torn about it.
I have had cats my whole life, and this is the first cat who he done this, although I had an aunt whose neutered cat also did this to her. Although it is disturbing to say the least, I feel bad to stop him since he seems to be acting in a natural way. All good cat owners know that you cannot rationalize with a cat the way you can with a human, so to punish him for this behavior seems wrong in my book. He has such little to do being a strictly indoor cat since we live in a city, and I try to allow him his little pleasures.
We play with him often, and give him a ton of attention, so it's not like he is really all that deprived by his indoor lifestyle. He loves to fetch, and play with the fishing rod toys, and will bat his toys around the house on his own when he is feeling frisky. At three years old now he is still relatively young, and much more playful than any other cat I have ever had, and many I have known. So this whole masturbating with mommy thing is totally friggin weird, but it's whatever at this point. It's not hurting him, and it's not hurting me, so I just don't care. It's already so unnatural for cats to be house pets that I refuse to be one of those people who demand human-like behavior and attitudes from my pet.
It is totally embarrassing, and I don't go around telling people, but like I said, doesn't hurt him, doesn't hurt me, and he doesn't get aggressive when I make him stop, so I'm just going to let him continue unless I find a good reason not to. I am definitely going to remember to ask the vet about it next time I take him in for a check up though.