if u were to end now, over this, to him it would probably humiliate him and crush his confidence to the point of despair, thats the fact of the matter. us guys place a lot of pride and emphasis on our "kit".
However, you're a young girl who wants to enjoy urself, i wudnt blame u for dumping him and havin fun with sum1 else. sex today, relationships tomorrow.
You got three options i guess;
1) Dump him for the penis thing
2) Dump him but blame it on something else (you think he's cheating, you hate his taste in music, whatever, its ur call)
3) Try some sex stuff thats inventive and stimulating so ur not just left in awkward silence with his penny whistle. Spice things up and make a go of it.
But for gods sake, be discreet.