ED refers to inability to have erection. You could have sex, which means you had an erection and could penetrate the vagina. So you dont have ED. You say, your first round of sex doesnt last. This means you are ejaculating very soon. This could be Premature Ejaculation if you leak within 1 minute of entry into the vagina. So if youw ant to last longer, you will need to learn the art of delaying ejaculation and this is best taught by a sexologist in a one to one consultation. Sex is emant to be a single good event. repeat sex is NOT the norm. people try to get a second chance but the penis are not designed to get erect at will. After any man ajaculates, there will be a time when a second erection will not occur, do what may. This is called refractory period and it is short in the teens and as age advances the duration increases. Rather than worrying about why 2nd erection didnt happen, it would be better to do it best at the first time.