hmmm, I think I have to disagree here. Sometimes people realize that they didn't like that version of themselves and change for the better. Or maybe they were promiscuous while single, but end up being very faithful while in a relationship.
Like I said, there are no guarantees either way but past behaviour can be good indicator of future behaviour.
Generally girls (other than guys) have plenty opportunities to have sex. If she refuses all of that until meeting Mr. Right (you) then it makes sense that she'll be able to refuse other guys while being married to you. Virginity is a sign of strength of character, and that is generally commendable, especially when in a long term relationship.
But you are right, people can change their mind about what kind of person they want to be. There are no absolutes either way, only indicators.