Pornography is not created in a vacuum. There is a large industry surrounding it and, at least according to some, some of the practices of parts that industry is troublesome.
There are also reasons to believe that watching porn might in some scenarios affect the viewer in negative ways that abstract sexual fantasy does not.
Reasonable people can of course disagree to the scope and accuracy of the above objections, but I'd consider neither fundamentally unreasonable.
Masturbation on the other hand is almost universally considered a normal and healthy part of adult sexuality.
Basically just because someone might find porn degrading to women or morally objectionable, doesn't mean they in any way oppose sex and their own sexuality.