I've experinced the same problem with girls I've dating for a time..!! I'm 9x7 and have been told that I make their stomach and back hurt, sometimes for days after sex. It's not easy to always be gentle, but try everytime..!! The girls often want it a bit rough, even they know they will get pain after..!! For me the problem is, I'd rather do it gentle for several days with good sex, than one day with rough sex and several days without sex...!! ;-) Well, most of the time... Sometime it's heavenly to be rough, when the girl wants it that way!!! ;-)