Underside of penis gets red and irritated after masterbation? std?

Not unless hand: Is infected. Force of stimulation, lack of lubrication, 'friction-burn' all more likely. If you were to repeat the action you use on your penis to your arm, you would likely see a less severe, but similar result--simple friction. The tissues of the penis are very sensitive (but you already know that!) and loaded with small blood vessels that can be injured when abused.