Casual sex replaces prostitution?

Challies has this sad and thought-provoking reflection from SuperFreakonomics:

The authors draw out all kinds of interesting conclusions about prostitution and especially about how prostitution has changed over the years. For example, they show that the wages for prostitutes have fallen drastically over the past hundred years. The reason is pure economics and goes back to the law of competition. "Who poses the greatest competition to a prostitute? Simple: any woman who is willing to have sex with a man for free. It is no secret that sexual mores have evolved substantially in recent decades. The phrase 'casual sex' didn't exist a century ago (to say nothing of 'friends with benefits'). Sex outside of marriage was much harder to come by and carried significantly higher penalties than it does today." In other words, in decades past women held closely to their virginity and were unlikely to give it away to anyone but their husbands. Today a man has, in the words of the authors, "a much greater supply of unpaid sex." According to the laws of supply and demand, prices must then fall. In our generation only 5% of men lose their virginity to a prostitute; in days past it ran as high as 20%. Today more than 70% of men have sex before marriage; in days past it was just 33%. Premarital sex has proven a free substitute for prostitution. Once the domain of the professional (at one time one in every fifty American women in their twenties was a prostitute!) premarital sex is now the realm of any woman. This has driven down wages through a strange but sad kind of free market force. I guess this gives us something to think about the next time we hear about the falling levels of prostitution. Though we rejoice when prostitutes find another line of work, it does not necessarily mean that we have cured one of society's ills. It may point to changing market forces based in turn on declining morality.