I need help with a mans .?

kk so i am 17 my hormones are very high i do get my s out of no were i get s wen thinking bout something y n all dat n i am still a virgin. but i would like to know y wen im with my gurl n we get into it we start kissing n i get a ***** she touches meh n im extremely hard but deres a twist to it even dough i get hard with her n everything wen i go to pull her pants down n shes naked it goes down i need to know y it does dat?? at times i have s for 20 to 30 minutes by it self . but wen i go to pu it in it goes down i am not gay i really love her n would like to know y dis is happening to me so dat i can fully give hur my love n make love with her????