Fucking pissed

My son's girlfriend came over and he was doing her make up. My son is a very artsy kid and he sees make up as art so he learned how to do make up and us honestly better at it then I am. My fiancé was having an attitude for no reason today and when be saw my son doing his girlfriend's make up he said "Fucking fagot". I told him to shut the hell up and he told me "Your son's a fucking fagot". I don't understand what his deal was. First, that word is so disrespectful to the LGBT community. Second, it's so fucked up to call him that just for doing his girlfriend's make up. I told him this and he said he doesn't need a bitches opinion. Nobody did anything to him to act this way towards us!