In truth most young men have at least thought about the subject you describe. Since your friend came to you in confidence after your reply, as opposed to embarrassing you in front of your other friends he will more than likely drop it if you do. We all have things in our past we wish we could change, and the key is not to let one event guide your life.
Thank you for your advice. So I should just let it go and don't talk about it again ?
You don't have to justify yourself to anyone for who you are/were.
It doesn't affect their lives in anyway.
But if you think you should say something, just tell them that it was a while ago, and it's not who you are anymore and that you have moved on from it and so should they.
OR, since I'm a sarcastic person, I deal with things with more humor than anything, I would play up to their suspicions and play a joke on them.
but, that's just me.
whatever you decide to do is your decision. but just know that if they are gonna make a big deal out of something that happened a while ago and had/has no affect on their lives, then they are the sad person with nothing better to do.
You haven't done anything wrong either just ignore it or if your still worried laugh it off and just say that fooled you as if you was playing the prank