Right they have to prove that she's a virgin, in otherwords, prove she hasn't had sex with other men. If she's not one then she's stoned(because she's been promiscous). God views sex outside of marriage as very bad.
I'm not sure why you are having so much trouble understanding this process. The family must produce physical proof of the girl's virginity, otherwise she is brutally murdered. I don' t see how promiscuity is even a factor. There are lots of reasons a girl might not bleed her first time. I certainly didn't. Should I be stoned to death?
Also, how can a process be considered just when the accused has to prove they DIDN'T commit a crime, rather than the accuser having to prove they did?
Anyway, I understand you are personally committed to believing that brutally murdering a girl on her wedding day if her hymen doesn't seem to be intact is "moral", although I find that very disturbing. Am I to understand, then, that you believe Jesus was wrong to put a stop to one of these "moral" executions?