Jealousy can be a pernicious force in a relationship, but I think it's quite a useful "marker" about the state of a relationship.
When people say they feel no twinges at all about their partner meeting an ex, or developing a burgeoning friendship with a new female, I think it is possible that they have checked out emotionally from the relationship themselves some time ago and are looking for an exit route so that they are not the "bad guy".
I also think that people frequently ignore their "inner voice" or the twinges they feel, because it is now culturally unacceptable to admit to feelings of jealousy. So people try desperately hard to be "cool" about these situations, even if instinctively, they feel threatened.
It seems far healthier to me - and realistic, if we acknowledge that a situation like this can be threatening, but honesty and transparency will nullify the threat. So a couple in tune with one another will each understand there might be a nervous twinge - and do everything in their power to reassure and behave openly about their friendships. They also understand that the lines in such friendships can become very blurred and that there are some warning signals to heed, such as the ex declaring that sex was great, wasn't it? That they are unhappy with their current partner and have fonder memories of the earlier relationship.
The safest friendships outside a marriage are the ones that wish the marriage no harm.
Meeting any friend who wishes the marriage harm and actually wants to sleep with you again is, I'd suggest, a threatening situation.
And it is a world away from the life-enhancing, non-threatening friendships most of us have with men who don't want to sleep with us and wish our marriages well.