Hi,
One of my past boyfriends had a similar problem but he wasn't even able to get it up to fuck let alone where I could suck and tug it.
We basically got around it because I let him know I wasn't really interested in his cock - if he was getting me off (and it's easy to do that without a penis!) I was quite happy no matter the implements.
It ended up that after awhile making me cum all over the place he forgot that he had problems, he was able to join in and receive some of the pleasure he'd been giving me. It was awhile longer before hand jobs and oral sex where effective but that happened as well.
I think it was a combination of an initial physical problem (epilepsy medication) and this then exacerbated by worry over performance issues - the amount of pressure that is attached to owning a cock - oh my goodness! I have to feel for you guys sometimes.
It can be hard finding a girl willing to leave your cock alone though - alot of self-esteem issues are then brought into play as she may feel she's not satisfying you, or sexy enough for you. You just need to re-assure her that you find her sexy as hell, and want to be sexual with her and do yummy things to her and leave not think of your cock for a bit - maybe(hopefully) it will just come to the party all of it's own accord.
Something else to consider if you do feel it's a sensitivity issue is getting a piercing or two - that might sensitize it right up
I don't know any men with peircings who weren't sensitize beforehand but I do know girls with genital piercings who say they weren't sensitize but now they are. Since it's the same kind of tissue it may be an option to explore.
Good luck.