Am I Crazy?

For not wanting my best friend to get with the ex I gave my virginity to? I am happily married to someone else now and wouldn't have it any other way, but at the same time, my ex and I are on good terms and I will always care about him because he's the first man I gave everything to. We were deeply in love and our relationship was intense and passionate, but eventually I came to the conclusion that he wasn't "the one". 
Now my best friend has a serious boyfriend but she's Facebook messaging my ex and flirting hardcore. She's a virgin and has been talking to him about sex and I kind of feel like she wants to see if she would be better than me at it and who better to judge than the one who first had sex with me? 
I have tried to explain to my ex why I don't want him to let this go on, but he doesn't seem to understand why it bothers me, and this is making me wonder..... Am I crazy for not wanting them to try anything? I just don't think I could be friends with either one of it happened....... 
By the way, in her Facebook messaging to my ex, my friend has been saying stuff like how she was jealous of me when I was with him and how she wanted what I had, but she never even hinted at that to me..... And she doesn't treat her boyfriends very well and I don't want him to get hurt again, which I know is what will happen with her..... Obviously she isn't on the up and up if she's talking about stuff like this to any man when she's in a committed relationship with someone else. 
Edit: I know to a lot of people, losing your virginity isn't a huge deal, but I grew up in a Christian home and was taught that sex before marriage was wrong, so at 20 when I lost my virginity and had sex, that was a huge deal. That's why this is such a big deal to me now.