Supposing the porn you're watching is a home movie of you and the wife?
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This is a sin for two reasons:
1. Even when married, a JW must deaden sexual desire (most do this very well),*
2. time spent making a home sex movie is time stolen from WT. You could've been out in field service, building an assembly hall, reading the latest WT, or going over your finances to see how much more money you can give to the GB so they can add more amenities to the Warwick Country Club.
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* sex is wrong unless you are a pedophile elder. In that case you can rape all the children you want, so long as there are not two witnesses to the rape.