"Benevolent sexism" is when you attempt to compliment a gender but inadvertently imply they are inferior in some way, or build up existing harmful stereotypes. Some common examples are things like

  1. "A woman would never do something that cruel" Are women incapable of cruelty? Are all women the caretaker?
  2. "Women are the more mature gender" Women have to be the more mature one?That's their duty/role?
  3. "I'd always believe a woman over a man in these situations" Women are incapable of being manipulative or being deceitful? Women never have ulterior motives other than some comic book definition of "what is right"?

And so on.

To be clear, this is not just "You can be a smarmy devils-advocate for any pro-women take". There are many statements that do not fall into this bucket.

  1. "Women are just as capable of STEM work as men"
  2. "Women's contribution to history deserve more credit"
  3. "Women deserve equity"
  4. "Girls should be encouraged to pursue their interests/passions"

Edit: And don't get me started on pro-women takes that are actually just anti-man