I think we start by not laughing — not at the 'man in a dress' joke, not at the boy who cries. Stopping our own passive participation in the mockery is the first step. Then we can actively model: I tell my son I love how gentle he is. I say it in front of his friends.
We can talk explicitly with kids about why some things get called 'for girls' — and how that's a put-down, not a description. When children understand that femininity is being dismissed, they can start to push back on it themselves, rather than absorbing the message uncritically.
For me it starts at home — making sure my kids see me valuing feminine things, not apologizing for them. If I'm excited about a sparkly decoration or a pink sweater, I say so. If my son wants to paint his nails, we do it together. These small things add up.
I've started gently redirecting when I hear kids dismiss something as 'girly.' Instead of letting it pass, I'll say something like 'I actually think that's really cool — why do you think people say it's only for girls?' It opens a conversation without shaming the child.