Oh dear the bulge in your pants has got bigger. Drop you pants now and show me you cock. You naughty boy you have got hard and I told you not to. I don't care that you can't help. I don't think you can be trusted so I am going to give you a test. I am going to let you stroke that huge cock of yours while you look at me. You stroke as fast and as often as you like but you are not allowed to cum. If you don't cum I will reward you by letting you lick my pussy. However if you do spurt you will be in trouble.
Now start stroking.