What is causing penis friction burns after sex?

Need lubrication. The friction burns, or abrasions, come from rubbing of the shaft of your penis against the vaginal wall of your partner. Be sure there is sufficent lubrication, either from foreplay or a water-soluble lubricant (e.g., KY jelly). Also, be sure you are gentle enough. Finally, if you have signficant abrasions; you might refrain from activity until you are fully healed.