Re: What the hell is going on?
I'm really sorry you are having this problem. I know it's got to be frustrating to you. The one thing I can tell you is that before my husband had a vasectomy, we used condoms for quite some time. He told me often that the condoms really did make him lose the sensitivity in his penis. I also noticed that every time we used a condom, sex lasted quite a bit longer and also felt that he wasn't as hard as he was without the condom.
If both of you are in a commited relationship and sleeping only with each other, you might consider taking some tests to make sure neither of you have any sexually transmitted diseases or HIV, and make the decision as to whether or not you feel safe using other forms of birth control such as the pill. I suggest this ONLY if you both are in a long term, monogamous relationship.
Another option might be the female condom? I've never used one, but it might be worth your gf speaking with her gyno about. This way, you still have protection, but you won't have the pressure from a condom on your penis. It might reduce her sensitivity though, so I'm not sure if it would be good for you guys.
Are the condoms you are using too tight? You might want to think about a little bit bigger size, or they even make the ultra thin/ultra sensitive ones as well. My husband preferred those, but they still did cut some of the sensation down, but not as much as the other ones.