Narcissist ex makes current nsa difficult & I don't understand why. Help?

If you are involved sexually with your baby mama & you know for a fact she isn't the jealous, clingy, delusional, intrusive, toxic type of female. Who accepts that y'all only still just bang with no strings. Why is it nearly impossible for the guy to just be honest/transparent, fun, etc; instead of still acting & doing all the fuckery he did in the past to kill the relationship? For example: lying about having intimate convos with people he interested in for whichever motive. Or denying and being combative about nearly everything. When none of that is even needed
Narcissist ex makes current nsa difficult & I don't understand why. Help?
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