I think it's a bit of a mistake to use broad brush labels like, "perverted."
Everybody fantasizes when masturbating. Everyone. And the nature of those fantasies are different for everyone. But perhaps most importantly, the linkage between those fantasies and our sexual behaviours is, for the vast majority of people, tempered by two things--the corresponding behaviours of sexual partners, and sexual ethics. For example, power and dominance are frequent aspects of sexual fantasy--but when these are translated into sexual behaviour, it is always within the construct of ethical sexual behaviour: safe, sane, consenual.
So, for my part I subscribe to Rule 34 of the internet: if it exists, someone has made porn about it. There is nothing that human beings cannot sexualize in some fashion. So whatever you, personally, choose to masturbate to, it cannot be described as perverse.
What would be perverse is acting out your fantasies with an unwilling partner, with a partner incapable of expressing consent, or engaging in actions that no rational person could be expected to consent to. When your behaviours, rather than your fantasies, cross those lines, that is the only time that perversity enters the sexual lexicon.
In my view.
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--James