yeah...sounds like desensitization from watching so much porn...the body gets used to it. Psychologically and physiologically. If you fuck the same bitch forever, you'll be bored in a few years and you'll be able to fuck for hours without cumming. If you watch too much porn and whack off all the time, when you fuck, your body and your brain will not detect that as the stimulation he is expecting.
Is like a self induced deca dick. Although 30-40 minutes is nothing...if I cum in 30 or less I have to take a break and screw the bitch again.