Let's take the classical trope of the "damsel in distress": A woman gets kidnapped, and a man has to rescue her. Is this trope sexist? "Of course!", says the feminist. It reinforces the notion of women as being weak and fragile, incapable of defending themselves, and having to rely on a man to help them.
Does this reflect the actual, real world, on average? That doesn't even matter. Works of art should be used as tools to change people's attitudes, not reflecting reality.
But never mind that. Let's just grant that the original trope is sexist. How about the gender-reversed version? In other words, a man gets kidnapped and a woman has to rescue him. Is that sexist?
Of course it is. But not in the opposite direction. In the same direction. You see, this panders to male sexual fantasy. This has undertones of SM and dominatrix fantasies. The woman is nothing more than a tool to excite the man's imagination.
What if both the kidnapping victim and the rescuer are women? Is it still sexist? But of course it is! This once again panders to men's sexual fantasies because this scenario is hot!
If the kidnapper is a male, it's obviously sexist (because it reinforces the notion of men being in power and subjecting women to their will.) What if the kidnapper is female? Again, panders to male sexual SM dominatrix fantasies.
Ok, but what if the victim, the kidnapper and the rescuer are all men. Surely there can't be anything sexist in this scenario?
Haha, how wrong you would be to think that. Of course it's sexist! There's a severe lack of female representation in this work, and thus it reinforces the notion of a male-dominant society where only men do things and act, while women are confined to their homes, out of view.
You may think that I'm straw-manning the issue. If you think so, you haven't watched enough feminist videos nor read enough feminist blogs.
(Yes, not all feminists are like this, not even the majority. However, some of them are, and unfortunately they tend to be very vocal and obnoxious.)