However as you grow older, you will wish you had your foreskin back because you lose sensitivity as described. See this: http://uncutting.tumblr.com/read
Bottom line, you will not give your GF an ogasm by pounding it in anyway, so stop worrying about it. Just focus on stimulating her clitoris orally or manually to get her off and she will think you are a GREAT lover. She wants her ogasm and pounding it in for hours will NOT do it for her. For the same reason, size does not matter.