Slavery: A series of erotic games (book 04)
Sean got on his hands and knees with his legs slightly apart so Ron could kneel between them.
"Mistress Shasta," I said. "I'd like to make a suggestion if I may, for the comfort of your slave. Ass fucking stimulates the prostates of males, often giving them erections, even if unwanted. Unless he's been trained for ass fucking, his cock cage may give him unbearable pain. I also recommend lube for both his ass and Ron's cock since he wasn't prepared like you for an ass fucking, with plugs. If he gets an erection during his fucking, I suggest Rhonda suck his cock and take his cum for him so he doesn't get it on the floor."