Is it normal that i cum extremely easily?

I can handle barely any stimulation before I cum. When I masturbate, I usually cum with a single stroke. Actually half a stroke, since I'm ready to cum when I go up and am cumming when I go down. I've had sex twice, and both of them were really embarrassing since I came as soon as I put the tip in her. I've read that I should try getting it to the point just before I cum and trying to hold it there and eventually I'll start lasting longer, but I can't do that because the amount of arousal it takes for me to start getting an erection is only slightly less than the amount needed to make me blow my load. Sometimes when trying to give myself an erection, I'll cum before I even get an erection. Has anyone else had this problem?