For me, it's an issue of degrees of separation. Someone asking for "shady historical characters" (and I'm assuming when you say "characters" you mean fictional people in historical settings) or extreme underage might make me think "... welp, glad I didn't match to them!" But ars_belli tried to nominate Nazi RPF for Yuletide at least once, IIRC - when it's entirely possible that relatively direct descendants of their victims are also participating. That, to me, is very obviously more fucked up than either of your other hypotheticals, and is such a different scenario that I don't think "hypocritical" applies to being bothered by one and not the others.