Hi there, There's nothing called as a 'retractile testis.' If you have doubts about it, kindly get examined by a general surgeon. About the size of your penis, and other queries. The size that you've mentioned is good enough for penetration and having successful intercourse. Masturbation has got nothing to do with sperm count, it's a healthy activity, as long as you're doing it when sexually excited. Make yourself aware about the art of foreplay, to satisfy your prospective partner. Size has got little to do with it. Good luck.