So I think there are a few reasons you hear negative things about marriage.
Partly I think that the discourse re marriage is slightly effected by comedy about it. That I think originates from the power that men give up to the woman in their life. Men want to appear as powerful as possible in a heirachical sense and so they talk down the power that the wife has over them. So instead of saying "sorry lads I can't stay for another pint, I love my wife and she rightly expects me home to help with the kids we co-parent. I might even prefer her company" They say something like"sorry lads, got to go, ball and chain's gonna have my tea ready"
Every emotionally evolved man knows that any power a woman has over them is entirely given.
Then there are those bitter after a breakup and serial divorcées who likely wish to blame others for their failures. The defence of their own ego necessitates them finding the fault was with the other. They are motivated to broadcast their virtue by denegrating either the institution of marriage or women generally.
Then there are those who regret agreeing to share their wealth with their ex wife. Those who don't understand the project of marriage or regret their mistakes on that front and chose to externalise the issue. Or they bought the nonsense about women and men being the same, not taking into account how much different it is for men seeking new relationships post marriage Vs being a single mother. Etc etc
Basically it's a lot of cope.