i have been masturbating by rubbing my penis against mattress without erecting it till i get the sperms out. i do it almost everyday for the past ten years.. i have normal erection (if i see nude pic) for 5-6 minutes...i have never masturbated with erect penis.. i want to know if il face some prob in future...i have no pain.. should i stop masturbating this way.. please help i have posted my problem at many web no one has replied.

I have seen several rumors and scientific-looking surveys on the internet portraying claims that masturbation face down could be harmful. The argument of the rumor-propagators is that the urethra which lies just above the undersurface of the penis gets bruised as one rubs his penis against a surface like the mattress, which is much harder than the hand. A few argue that this way of masturbation causes less strong erections. However, I do not know of any scientific evidence that these claims indeed are true. 

What I do not understand (and do not buy) is your claim that you have normal erections when you see nude picture and yet manage to masturbate to ejaculation without erection. Your penis seems to have a split personality?