I have a lot of girlfriends who do constantly. I often know all of their bfs/husbands faults, and shortcomings.
Growing up, my parents had 2 rules that I follow to this day.
1) Never complain or say something negative about your partner if they aren't their.
2) Never disagree with your partner in public
The last one can be hard, but we both try our best to keep the other's thoughts in mine because of this rule.

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