See, the story goes that it was part of their initiation ritual - the osculum obscenum is the Latin euphemism I’ve heard for it. It was supposed to prepare them for possibly suffering the worst tortures and humiliations imaginable (though given medieval Europe’s ingenuity at developing new tortures, I’m sure things generally got much worse than that for the folks the Knights were waging war against . . . .) The story as I’ve heard it has them deliberately commiting blasphemy during their initiations as well, in order to be able to lie about their beliefs in potential times of need later on.
I don’t know enough about them to evaluate these old ideas. My assumption tends to be that these claims were probably just slander designed to stir up opposition to them. But I have to be curious if there’s any truth to it - or even if there’s any historicity to the claims, rather than their being some sort of modern invention.