I think it has levels, but it's more of a branching effect then a linear progression. So you can slide sideways on levels or go up or down. I'd say general levels might be classified by roughly what percentage of the population participates in the activities depicted, ie how 'taboo' it is. I do think that it would be rare if someone skipped a level, and say, went straight for scat...

i would think you'd have to be pretty comfortable with most of the stuff on one level before you started looking at/for anything in the next one.
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victoria secrets stuff, no nipple

topless/nude

'normal' sex (and might as well add oral to this level)

toys/anal/facial/'rough'/bondage/fetish

watersports/"rape"/scat/ultrayoung/snuff

hey, don't lump watersports with rape and snuff. That's bound to get some people pissed (no pun intended).

EDIT: OK, the pun was intended.

leave it to Art to make my head hurt from thinking too much:D

Very good pun.

Scat = crime against humanity.