The word nasty is really just the effect of sexual repression in a conservative BS society....
wild uninhibited sex in which people do what they really enjoy, rather than what Dr Phil, the pope, the president their parents or their teacher thinks they should enjoy, should be a standard, a constant, not nasty.
words and understandings like slut, nasty, perversion, ETC are total BS. their meaning is completely subject to the opinion and views of the speaker. look them up. nothing senseful in a definition.
Do what is safe which you both (or all 3 of you, or whatever) enjoy and choose to.