This may develop into a fleshed out story, but I've had this idea where my cock continues to grow until I cum, maybe due to a new medication? It starts out innocent enough, a woman giving me a bj, when she realizes my average size penis keeps getting bigger by the minute. Soon I'm too big for her mouth and, pulling me out, she watches in awe as my cock grows, inch by inch, in front of her eyes. I then fuck her, stretching her pussy more and more with each thrust until she can't handle it and has the most intense orgasm of her life. This puts me over the edge as well and I fill her with a huge load of cum, after which my cock then returns to its natural size.