I Spanked Bare Bottom

My son's penis is I would guess about 4" give or take whenever he is erect.
It is a turn on when his hard little penis is rubbing on the naked flesh of my thighs. It's lick a small electrical shock on my skin when his little hard-on is riding against my skin. As I administer his punishment spanking his penis rubs up and down along my leg and his balls are drawn up into him like he's about to cum. He clenches then releases of the muscles of his bottom and it's so cute to watch his brown ass-hole to peak at me and then disappear again.

Last time I spanked him I had to go into my bathroom and relieve my tensions thinking about that beautiful hard penis that he is carrying between his legs.
I can talk to any of my friends or family about this because they would think that I'm an evil Mother. I could use your advice on this. Should I stop spanking him as a punishment? Should I let him wear his undershorts when I spank him? If I let him wear his boxers I will still feel his erection on my legs. Should I have my daughter in the room when I give him his punishment? His father is no longer in the picture so he can't help. I could use your advice, I don't want to do anything that I will regret or ruin my babies future's