Dude  be  realistic, wht  evn if she had sex with her bf.She is in a relationship with you at the moment and she is true  to you, she is not  cheating  or  sleeping around, thts  what matters.I didn't wana call you stupid but  you sure  do sound like one.Get over it  evn if she had sex  with her ex, it was in the past when he was her bf.However, if you can't accept her as  the way she is,and if this thing is taunting you,you are  better off single than being in a relationship.
                Lastly, i think you need to grow up.