These categories aren't supposed to be hard and fixed, just convenient for the reader to find something of a general tenor.
I'm glad you said that, because I had assumed, but never had it explicitly stated. For example:
Consensual sex. Because it's in let's say one-on-one rape, doesn't mean neither the victim, nor abuser can have consensual sex. That would be silly, but as you pointed out we also have story codes so people can search for fetishistically triggering material. (They want to read about mother/son incest, they can search for it.) Also, so that if anyone has certain subjects which are phobically triggering like mother/son incest, they can avoid it, because it's marked mother/son incest.
I quite often use triggering (Triggers work both ways. Arousal is another triggered response) events like mother/son incest for motivation. An Anger Retaliatory Rapist subconsciously hates women, because he was molested (And otherwise emotionally abused/dominated) by his mother. So, I could put it in Mixed Groupings, or just put a Trigger Warning in chapter heads, but again if someone wants to read a story about a serial rapist, serially raping women, it's generally better to put it in that section.
And finally, an issue that hasn't been brought up is Spamming. These boards are fairly high turnover, so every time someone posts in 1o1, it pushes everyone else's posts down in the stack. If you have a Mixed Groupings story, then spam it in 4 different boards, you push everyone else's stories down on all 4 boards. So, this can be abused. Not accusing anyone of that, but we have rules to prevent that sort of thing.
