I've had 3 colonoscopies, starting from when I was almost 39, so from experience i can tell you:
You can leave your top on under your gown. They only need the bottom half of you exposed.
They are testing for polyps. If they find them, they will extract them from you during the colonoscopy & they will be tested to see if they are malignant or benign.
I was given anesthesia & knocked out. Afterwards, they offer you something to drink & you will be farting out all the air that's in your colon. They assure you it's natural & not to worry about doing it, especially because it can create some funky noises.
Some folks insist on being awake during it. I never have.
Having to undergo a prep to clean yourself out the day before, I don't think you'll be in a condition to show arousal when you have your colonoscopy. Although I could be wrong. i don't think they'd be able to see anything since you're rolled to your side & that the front of you will still probably remain covered by the gown anyway. Again, i could be wrong if they move your body around.
I got rid of a lot of hair between my ass cheeks to make the area easier to clean, & to minimize & avoid dingle berries that could get caught up in the hairs. I think observing that you shaved there is the least of your worries, although that could contribute to the irritation.