This is not about insecurity or openness. Its about what constitutes infidelity for men and women?
I have a (unproven) theory that, in the heart of their hearts, men fear sexual infidelity much less than they fear emotional one!, while on the contrary, women fear sexual infidelity relatively more!
The idea is that men, unlike women, believe (and rightly so in their case) that uncalled for sexual acts are a spur of the moment thing. It isnt necessarily a display of their intention to betray their current partner, or any symbol of being dissatisfied with her.
Its like, "...i am really sorry honey, this hot blonde just kept looking at me and I was 5 beers down, and trust me she wasnt even as hot next morning, ....you are still the prettiest!"
So, men would be rather forgiving of their women committing adultery and apologising for it, than if their woman was seeking emotional support and security from another man. That, inherently, "writes him off" as her man!
(So, at a not so accurately simplified level, 'he is ok if she kisses him, but not OK if she laughs with him!' )
...that girl is NOT mine because i do her!, she is mine because she trusts me and finds support in me and feels at home in my arms! if she finds these things somewhere else.. i am not her man anymore!
The unfortunate (or maybe not) bit is, that women correlate physical intimacy very strongly with emotional dependance. Which leaves the poor man wondering as to what exactly is she doing? Seeking emotional support or seeking physical satisfaction?
The probable fact is that, 'he' might betray physically but not emotionally, but when SHE betrays, she does it EVERY WAY!