PJB does NOT belive that Islam is a "more" moral system--but he DOES believe that the typical (not nutso) practice of Islam has prevented moral decay.
By the way, it's hardly limited to sex. Islam would have very nasty things to say about, ohhhh--Jeff Skilling, and his ilk, as well.
In what sense?
Slavery still exists in most Islamic countries.
Adultery still exists in all Islamic countries.
Bastard children still exist in all Islamic countries.
Rape is still common in all Islamic countries.
Murder is still common in all Islamic countries.
Theft is still rampant in all Islamic countries.
Oppression of women (no education, no voice in society, no choice of marriage) is still common in all Islamic countries.
I guess technically you could say it hasn't grown worse since things are still the same as they have always been in the vast majority of Islamic nations: Might makes right and the voice of the prophet (i.e. the person in charge) is above petty things like "laws".
But does that matter? After all, some trailer trash girl pointed at a prisoner's privates, so this must mean that we are in a worse moral situation than Islamic countries, doesn't it?