My sisters and I were spanked mostly by mom until our early teens. We always had to at least bare our bottoms and lay across her lap. All was fine with that until I reached the age of noticeable boners.
Normally, I was so scared about the spanking that any stimulation in the genital area made no difference. However, as I grew older, and so did mom, the spankings began to sting less and less, and I began to fear them less and less.
Mom would lay us on her lap and lecture for a bit and then start spanking. Then, one hot August day, I was on her lap, rolling my eyes at her lecture, when I realized that my bare cock was on her bare legs. Now, I was fully aware that she was my mom, but apparently my teen-cock did not know that. I began to get a bona fide boner right there on her lap. The more I tried to repress it, the harder it got.
I was totally embarrassed, but mom made no notice of the engorgement. I knew she had to feel it, although it was still probably no larger than 5'.
Then I realized that she did notice. Right before she began spanking she parted her legs until my cock popped down between them. Then she closed her legs again. At this point I all but forgot about the spanking. Well, not totally. I quickly figured that with every swat I could feign a reaction by thrusting my cock between her knees, as if I were pulling away from the strikes. This felt so incredibly good that I could not stop.
Within five or six smacks I started cumming. Enormous and forceful ejaculations that were so strong that I didn't care if she noticed, or not. About the time I was drained, she stopped the spanking. I just lay there for a moment, letting the last of the cum dribble from my boner onto her calves. I looked down and there was a puddle at her bare feet on the wood floor.
Finally, she told me to go to my room, and I slowly rose, trying to hide my still erect cock from her view. I did not look at her, and she did not say anything. As soon as I was standing I turned my back toward her and left the room.
Afterwards I felt incredibly guilty, but highly stimulated at the same time. I had used my mom's legs for masturbation, and she never said a word.
I suppose it's a good thing that that was my last spanking.