I want to understand why men don't just stick up for themselves?
Well, we can't "just stick up for ourselves," because that's against the law. If your FIL's club refused to admit women, the state or the feds would shut it down.
How has life gotten this gay?
Because politically, women + simps outnumber men, and we live in a democracy. The provision in the Civil Rights Act that destroyed most sex-segregated spaces (including, most recently, the ladies' changing room at the gym) was inserted as a poison pill, the expectation being that no man would vote for a bill that banned businesses that catered to or hired only men--or, for that matter, only women. Turns out, there were enough simps in both houses that the bill sailed on through, becoming the law of the land. It is now illegal in America to hire only young women to serve as airline stewardesses or for a lingerie department to refuse to fit a man for a brassiere.
Now, you might be objecting, "I wasn't talking about businesses! I was talking about clubs!" However, think for a moment what a business is. A business is, fundamentally, a social organization. Which sex created just about every major industry and corporation in the world? Men. There's no daylight between male socialization and business, because male socialization tends to revolve around creating hierarchies to accomplish tasks. Whether we're camping, going to war, hunting, making money, or playing dorky elf games, it's just what we do when we get together.
The social reasons why life has gotten this gay are virtually infinite. However, one political force for this is the kind of simping male who craves the constant presence of women. Even marriage and regular pussy access doesn't seem to cure him. Around 20 years ago, there was a law being proposed or discussed, it may have been this very law, and my current internet discussion group and I were discussing it. I was firmly on the side of men being allowed to socialize. Several of the other men in the group, a number of them married with children, sneered things like, "any man who feels like less of a man when women are around is insecure," or "maybe things you wouldn't do with your wife around are things you shouldn't be doing," etc.
Modern laws against violence means that simping males cannot simply be beaten into submission. They are given the vote, the same vote that any man who actually owns his balls has, and so they first voted to let women vote, then voted with the women to destroy all men's spaces. Thus the current age.
You're asking a group of men to explain the motivations of bitter rug munching hags.
Men understand women's motivations better than women do (e.g. why do women dress in a way that shows off their tits and asses; women say it's "for other women and has nothing to do with men," and believe this, but they are wrong), so it's a fair question.
Why do women(dykes)shit up men's stuff? I tried asking the dykes at man hate thread and all they could say was its men's fault bc theyre insecure.
One reason is women don't like feeling left out. This is seen in childhood, when certain little girls insist on being allowed to tag along with whatever the little boys are doing. She'll cry and whine to the nearest female authority figure until the boys are forced to allow her to play with them. It always seems to this sort of girl that whatever the boys are doing must be much more fun and exciting than whatever they're doing. This is why we now have two scouting organizations in America, the Girl Scouts and the Everyone Scouts. When the Boy Scouts were abolished, it was pointed out that the girls have their own scouts, and the women seeking to abolish the Boy Scouts responded that the Girl Scouts do boring stuff, while the Boy Scouts do cool stuff. A true and accurate observation, but the reason the Girl Scouts do boring stuff is it's run by women for girls, and women and girls like boring stuff.
Part of the above dynamic is because women achieve their social validation by getting attention from men. The girl who is liked by the most and best boys is the #1 girl in the pecking order, and the girl who can't even get boys to make eye contact is at the bottom of the heap. So there's a certain sort of woman who, when she observes there is a place full of men with no women around, instictively wants to go there so she can get male attention and defeat her rivals. This sort of woman is usually sleeping with one of the men in the space within minutes of her arrival.
You also have the "bitter church lady" dynamic. There's a sort of woman who looks at things boys do, like playing in the dirt, chasing each other with sticks, playing at war, and sees them as disgusting and antisocial. Boys, she thinks, should be more like girls. She is intrinsically biased to suspect that any time men get together without women to police them, mischief is afoot. She wants women to invade all men's spaces in order to keep us from telling dirty jokes and drinking too much.
Dykes like to invade men's spaces because it makes them feel like they're not "girly." Lesbianism isn't real, so they have to do absurd things like pretend to be "day laborers" to validate to themselves that they are totally butch and totally into girls (plus get the male attention they secretly crave).