What matters is the person to whom you’re referring to, not the others who may be in earshot (unless the insult in question would apply to them, too). If you call a woman a slut, you deserve what happens to you. If you call a sailor a squid, probably nothing will happen to you, but that sailor will put you on his list of people who are wastes of human flesh.

You know, in another time, refraining from insulting others was referred to as “common courtesy”.