Many might argue that sexual perversion is the predilection to be aroused by things that the definer sees as unnatural or unlawful.
I, however, am too much of a moral relativist*to subscribe to that. Instead, I define sexual perversion as having the predilection to be aroused by things that the pervert themselves finds unnatural or shameful. Indeed, to be truly perverted one the shame and unnaturalness is integral in the arousal.
So, for instance, a child molester might not be a pervert, even if they feel immense guilt over their actions, unless this guilt was in itself arousing.
Similarly, a truly bisexual person who, nonetheless, mainly bats for one team, would not be a pervert, but someone who occasionally wants to just be all nasty and forbidden and dirty with a member of the non-preferred sex is.
Finally, the classic “pervert” who sneaks peeks of naked or semi-clothed people or exposes themself again, is only a real pervert if they are aroused by the weirdness and wrongness of it all.
You can even extend it to non-sexual things: for instance, once in awhile I like to look at the goatse image, not because it’s sexually arousing (it’s not,) but because in my mind i’m like “ewww, thats filthy and disgusting and so wrong…lemme see it again.” So in that case, I like it in a perverted way, but not in a perverted sexual way.
*in the sense that, while I believe that morals are, indeed, left up to the individual, there are nonetheless ethical standards that should apply to all, since they are pretty objective and work both ways in most situations.