as i become erect my foreskin retracts and stays that way during sex, the lubrication of the vagina eliminates most of the friction that would cause the foreskin to move with each thrust. The foreskin is vital to protect the glans during everyday activity. I have read anecdotal evidence from men who have lived with and without and they experience huge loss of sensitivity because of the glans rubbing against their clothes. If as an adult you are considering it or your doctor is recommending it, just say no. If it was done to you as a baby, do not thank the perpetrators and don’t do it to your children, let him decide when he is an adult.