Ahem. May I suggest that, just for starters, they have no idea of the total number of sex offenders who attend? They only know the number of convicted sex offenders. Spend some time reading federal Bureau of Justice Statistics reports and you'll find that about (I seem to recall slightly more, but memory could be wrong) half the convictions every year are first-time convictions.
Just think of all the Catholic priests who got away scot-free for so long... just because you have no convicted sex offenders around doesn't mean you're safe. Ever.
However, if the church bars sinners, it would be empty.