When I was at camp I persuaded one of my friends to show me his penis and I would show him mine. We never got caught, but I feel bad about it. What should I do?
I know you are curious, but certain parts of our bodies are suppose to remain private. The general rule since the fall of man is that a person doesn't expose their genitals to others. We make efforts to keep our privates private. Of course there will always be times when it can't be avoided, yet we should make efforts to minimize it. Being seen naked in front of others ought to trigger a sense of shame. "Your nakedness shall be uncovered, yes, your shame will be seen" (Isaiah 47:3). One of the complaints God had against ancient Israel was the fact that they lost their sense of shame. ""Were they ashamed when they had committed abomination? No! They were not at all ashamed; nor did they know how to blush. Therefore they shall fall among those who fall; at the time I punish them, they shall be cast down," says the LORD" (Jeremiah 6:15; see also Jeremiah 8:12).
There are going to be times when you can't avoid being seen or seeing someone else naked. However, we should carry with us a sense of respect for the other person. We should avert our eyes, not making it a game of dare with someone else.
What do you do? Apologize to God, learn your lesson, and don't repeat your mistake.