Actually, I don't think a single sex event or venue is automatically sexist, although I know that is how the law tends to interpret it in some cases.
To my mind there are some good reasons for people of the same sex, ethnicity, cultural background and etc to want to get together, share what they have in common, celebrate it and continue the bond that being alike can generate.
Where it is wrong, and sexist, is when it is used to deliberately exclude people who are 'different' from the special privileges of membership.
When I was a kid, my father was refused entry to a local golf club because of his racial background. He didn't play golf so on that level it wasn't a bother to him. But, in the small country town we lived in, that was the main place businessmen met for the networking and semi-social activities which grease the wheels of commerce to a very great extent.
After Dad was publicly refused entry (despite going there as the invited guest of a member), he began to realise why he found it hard for his goods and products to compete in our town ~ the deals were already stitched up in Friday night drinks at 'the club'.
It is that sort of discrimination and exclusionism that forced women, people of colour and the lgbt community to push against the doors of clubs and organisations which presented as harmless 'social' groups, but were actually the places those in power went to avoid having to share with others.
Today, businesswomen and business people of colour are welcome at Rotary, and men are happy members of the Country Women's Association. But it wasn't always so.
The difficulty for the law comes in deciding which bastions of greedy privilege are just that, and which are simply organisations for the sharing of common bonds.
Because there's no real formula to help the law decide, it usually comes down to a simplistic case by case "unfair unless it's open to all" basis.
Most of us would realise that in reality that is not always the case (as some of your examples show), but for those who are somewhat obsessed with the idea that everyone else is somehow getting a better deal than them, the easy targets are always useful.
I'd like to see those type of individuals storm the barricades of the Swiss Bankers' Association, but then they won't try anything too hard, when they can just get their jollies by tormenting some local women or kids, instead.
Cheers :-)