Fellatio implicates sex. Solo sex_toy_fellatio is not sex because sex requires at least two characters, so sex_toy_fellatio must not implicate fellatio. The problem with implicating strapon_fellatio to fellatio is that a fellated strapon does not need to be attached to another character, unless an arbitrary and counterintuitive rule is made to the contrary, which is how the wiki is written. The utility of the sex_toy_fellatio and strapon_fellatio tags is that they describe distinct, taggable, pseudo-masturbatory acts without triggering a sex implication. I thought this utility was intended and imminently apparent--a classic mistake of mine--but it looks like just a happy accident.

Really, solo sex should not return anything, but tentacles are not a character and we have the multiple_* tags so that's "solo sex -tentacles -duo -group should not return anything". Some of the results are mistags like handjob (implicates sex) instead of penile_masturbation (+holding_penis for the solo "handjobs"), fellatio instead of autofellatio, and disembodied_penis posts mistakenly tagged solo.

If an act between two or more characters would normally be tagged sex, then two or more characters performing the same act with a sex_toy as a substitute should probably still be tagged sex. Interestingly, double_dildo sex -solo has just a few more posts than double_dildo -sex -solo, whereas I expected more bias toward the former. The proper tag operation to ponder is:

implicate toying_partner → sex