Some guy: "Hey, I feel this way, it's getting me down."
TSR: "Hey, don't feel that way, *******!"
Good advice guys.
OP I think that what you need to realise is that, if anything, your girlfriend will become rather desensitized to penises and will find, out of context, little sexual appeal from them. In any case, I'm sure she would be aware, first-hand evidence or not, that there is a massive range of **** sizes, and just seeing this range isn't going to make her entertain any thoughts of dropping a relationship over something so trivial. You might not stop feeling slightly uncomfortable, and if you don't, that's not something to be ashamed of, you can't help it. But recognizing that this feeling is just that, and not a product of evidence or rationality, should hopefully help you to calm it, and to avoid letting it affect your relationship.