It can be interpreted in many ways. The taboo relationship between a mother and son if mentioned in a sexual way, dominatrix's preferred name to be called, how some people actually call their mother, someone who needs a caretaking woman in their life, and even trickles into other places that this game probably won't get into. The safest way to ensure everyone is happy is probably with a toggle option. If "on", then the phrasing will be used. If "off", alternative phrasing can be used (ie Mistress, Ma'am, or even just default to the character name). It would be a few extra steps in programming with a logic gate, but other than that, should be pretty versatile.