If being in a relationship without insecurity is abnormal, then I'm happy to be a snowflake.
But I envy you living in a snowflake world without insecurities, cheating, divorces, etc.
Sometimes in life we develop defense mechanisms that are self defeating. I bet a lot of us are familiar with the conundrum of the IT department that always says "no." The environment becomes too constrained for work to be done, so the rest of the business develops shadow IT out of necessity.
Telegraph to your partner that it's unsafe to discuss their friendships with the opposite sex with you, and they will start hiding that information from you.