I thought retrograde ejaculation backed up the semen into the bladder?

All the same, if there is a preexisting scrotal injury of some sort, I could see how yanking on the little dude might be worse than sex with a lady. During masturbation, unless you are one of those fellas who uses oil, pulling on the shaft tends to shake the whole, uh...apparatus. During real sex, the shaft glides in and out (if you did your job right), so that the entire genital area isn't getting bounced around (depending, of course, on how you go about it).