If its unrelated to the crime, its not evidence, and its legal pornographic material, then they have no legal right to retain it. TBH I dont see how him having legal porn, or not having legal porn makes any difference to whether hes a peeping tom or not.
I seem to recall that some people claimed that Micheal Jackson having pornography proved he was a pedophile.. despite the fact it was ADULT and legal porn, which proves nothing other than he likes naked ladies, and that people like to jump to crass and incorrect assumptions.
Now if it were illegal porn.. that would be a different matter...
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"There is a time when the operation of the machine becomes so odious, makes you so sick at heart,
that you can't take part" [Mario Savo, 1964]