The reporting rate for rape has greatly increased over the past few decades for two reasons -- one good and one bad. The good one is that the stigma attached to victims has been greatly reduced (e.g. the suggestion that "she was asking for it" has become much less acceptable in both polite society and defense arguments). The bad one is that dubious cases in which a change of heart after the fact is reported as "date rape" have polluted the statistics.
Of course, an increase in the rape report rate makes the decline in total reports all the more striking, and all the more damning to the "porn turns men into rapists" theory.
How does one determine that the rate has increased, since there is no way of telling how many have gone unreported?