Staying pure is actually pretty hard (pure is definitely harder than not-quite-pure, but then again, choosing a "total slut" route ends up being quite hard too, because you start accumulating permanent debuffs at some point)
Rule of thumb: if you're in there simply for the lewd scenes, pick one of the two easier difficulties. If you want an actual gameplay, select normal (for your first experience) or higher difficulties (for once you're familiar with the game)
In this game, everything is a negotiation. Every choice has consequences.