Okay, I won't make fun, although I hope you realize how difficult that will be. On the other hand, it's a valid question for the sexually inexperienced, so I'll put on my Serious hat and try not to be too graphic.
Yes, I think you are being paranoid. If your hygiene habits are good, you have nothing to worry about. Some people prefer to wash their private areas prior to oral sex, and doing so might help you relax more, especially if your back end has, um, tended to other business since your last shower.
Unless you are specifically engaging in anal play, your butt is irrelevant to oral sex. I know it's the same general vicinity as your genitals, but if your guy has even a remote clue as to what he's doing, his nose won't be near your anus, trust me. (It's not like we're dying for a whiff in the first place.) The only exception to this is 69, which I suggest you avoid until you become more comfortable with oral sex.
Let me ask you a question: is oral something you want, or something someone is pushing you to do? If you aren't ready to do it, don't do it. Yes, it is fun and feels great, but don't let someone pressure you into doing it before you are completely comfortable and ready.
I hope this helps. Thanks for the question.
Two words..., "baby wipes." (keeps a travel pack in your purse).
I wish the bidet would catch on here.
I'm italian and I really don't know how you guys can live without it!