Well i heard that if you're using a condom that can make a guy loose it. Not sure why, but its really common. When me and my bf had sex it happened to him and he was really embarrased and ashamed, i told him it didnt matter and was most likely a mixture between being nervous, insecure and the condom- either that or the condoms too small and its cutting your blood circulation off. Hope some of this helps, this is all we found out when we were trying to deal with it as it used to really get him down.