I don't think it's a good idea, HOWEVER... I have known of some adult friendships that have tried the sex thing and they still remained great friends. But this is with adults who are much more mature than people in their teens or 20s. People that young aren't ready for a relationship like that and eventually their emotions will get in the way of a friendship.

I do not agree with sex at all outside of a relationship and I know if I wasn't married, even if I did think it was wrong, that I would not have the maturity to be able to handle that. Sex is a very intimate thing and can cause a lot of problems if you aren't ready to face them. If you really want to keep this friendship than I recommend not doing this. I am not saying that you are not mature, but from this post it is obvious that you are unsure and being unsure can cause problems as well.

Take care and I hope everything works out for you