I think the problem a lot of people here are having is that they're assuming the accusation is active sexism. Like it's a cognitive decision to go "phht, what would she know, she's a woman".

I suspect the vast majority of mansplaning scenarios are subconscious. They probably don't even know that's what they're doing abs would never see themselves as being sexist. I think that's because everyone sees the word "sexist" and associates it with clichéd extreme sexism, like cat calling, not wanting a Female pilot, ignoring their ideas in meetings etc.

The thing about subtle unconscious bias is that you're almost never aware you're doing it, but it still has similar effects on the affected group.

The healthy thing to do is to listen to the person it's affecting, analyse the scenario, and reflect on if it's something that you, or people you know, might have been doing without realising.