I’m so happy I found this thread, because our 2 year old, spayed female just started doing this to our arms and legs recently. I believe it’s a compulsive behavior, somewhere between self-soothing and dominant play. We have her AND her litter-sister—both were feral barn kittens before we got them @ 8wks. I believe they were actually younger than 8 weeks and prematurely weaned, as both sisters have exhibited unique behaviors that I’ve attributed to early-weaning. This same girl that mounts and bites used to nurse herself to fall asleep her whole first year. I’ve had cats my whole life and have never seen such weird behaviors. Thanks for posting—FWIW I believe this is a coping mechanism of some sort, but I’m certainly no cat whisperer or professional.
Kabuto used to do the same thing to me when I sit in the chair playing computer games but I don't think he is nursing.... it's more like he is err.... "pleasuring himself" he would even thrust at one point. When I shooed him off his willy sometimes was sticking out.