So, they are not sluts, after all. They are just doing it to make their husbands happy. And, you seem to think that a woman has to have all of this experience to make her husband feel like a man and happy in the bedroom. I don't know. I don't think that I have seen this line of reasoning before. It sounds like rationalization to me.
I think that if a woman is having sex simply for practice, she is abusing her body. Sex implies intimacy and should at the least be shared with somebody who you care for, inside or outside of marriage. To simply practice, cheapens the experience (a friend of mine is basically doing that right now---"practicing", and i'm quite disappointed). Women, unlike many (young, in particular) males, attach emotion and themselves to sex. It is important that they realize that not being with somebody you care for cheapens the experience and is detrimental to many young women's well-being. But I would not preach abstinence to them any more than I would preach to them to be wise in choosing a partner.