worse None of this is intended as a personal slight on any particular married woman. The level of confidence and trust between partners is an individual thing.
The OP started with asking the question of why was this definite exclusion happening? (Yes holds hands up it was childishly phrased).
First the obvious happened; the natural public bashing of "who do you think you are?...you're so vain/shallow/flirty/cocky ect as readers took a personal affront on the implication that they are afraid their partner would engage with a single woman.
However some posters have clearly given this a lot of thought. There are many anecdotal tales backing up this story including one from a male poster. Therefore it would appear this is more widespread than it first appeared and not as obvious as blaming it on the local vamp out for a man.
To acknowledge these experiences is the first step in making improvements. If all of us women live to our statistical life expectancy then it is a problem we will all experience later, if not sooner.
Verity, a character in Poldark (yes I know, how girly...) chooses to run away from the security of her home and family to be with a known abusive, bad tempered man because the alternative of the spinster life is so appalling.
Yes, times have changed and women may now travel, work and play as they wish but I think there is still a stigma and therefore an undercurrent of social exclusion for any single woman over the age of, say, 35.
To take this argument further, is this subconsciously known and is this one of the reasons some women will stay in an abusive, unhappy partnership rather than facing the social loneliness of being single at a later age?