Having a Goomar (sp) in the mafia is like peanut butter and jelly, they pretty much go hand in hand. When guys get a bump to Boss they dont say to their goomars, "hey, i gotta stop seeing you now".
Plenty of Bosses have/had goomars, gotti had many, hell even the Chin had one. So to say that running around with another woman is a disgrace when you are the boss is just not true. Also, he did not "flaunt" it to his wife, did she know? yes. Was he careful? no. But he certainly did not "flaunt" Gloria to his wife.
As that being one of the reasons he was killed, forgetaboutit. Big Paul got killed for ALOT of reasons but in the end the biggest reasons were his hatred for John Gotti and his overall greed.
I don't know if having a mistress on the side while married is a 'mafia thing' or instead it's just someone that's into cheating being in a bad or unstable marriage and they do not want to get a divorce because then their wife would get half of their assets, custody of children they may have, and may even get property they have?
Certainly not all people involved in this business have mistresses or cheat on their wives.
