Most Helpful Girls
I was in pretty much the same situation when I was 16. I had this dynamic relationship, strong feelings, first time sex, and of course I was telling everything to my best friend. Soon after I was with the guy, we broke up for good, because he was an asshole :) but the sparks stayed.
7-8 months later (we were not speaking to each other with the guy) and guess what - my best friend started an affair with him while she was in love with a mutual friend from the company :) She said she felt like they have a strong connection and may be both falling in love.
You know exactly how I felt :)
Now, 15 years later, I know that she was lying to me for so much things and that she is a very fucked up person. But this is her, not your friend. And I know she wanted the same that I had with the guy, and he was opened to everything with boobs and long hair :) Which may be similar to your situation.
So, my advice to you is to think about it a little more deep.
Is it possible that something may happen between you and the guy? Three years ago you were kids.
Or you think that you have feelings, just because your friend is interested?
This is important. If you still have a special and deep connection with the guy, and he feels the same, it's better that you clear up things for everybody.
If it's just nostalgia, let them try with your blessing. Believe me, it's better that you talk about it and everybody involved know how the others feel.
You are right, it will happen behind your back and this will ruin your relationship with your friend.
Speaking about it will not erase the uncomfortable feeling and the jealousy, but these are normal feelings and you should not consider them as special :)
So, good luck!
The question is what is the actual problem? I find it disgusting in knowing people have premarital sex, and then wants that with who you may have [if you're sexually active/and with that person] sex with at that time. Agh! It's like do anybody THINK anymore! But besides the fact. If your not comfortable you have to find out why this is. Either you still have feelings for the ex, or you know that your ex would not treat your friend right after whatever caused you two to leave. You shouldn't be having premarital sex at your age and even before marriage. Now you know one of the main consequences of premarital sex. This is not a good idea. And I highly suggest you speak to her from an objective point of view. Starting with your sex life with him and what happened. Because sadly, he's going to treat her the same way he did you.
Most Helpful Guy
You certainly can't control either your ex or your friend. Let them do whatever they want.
At the same time, set up personal boundaries on just how "close" you and her remain. Since she wants that relationship, I would refrain from telling her very personal things you might share with a "best" friend. Find other people/activities to spend more of your time with and slowly back away from her until the distance is more comfortable for you.