This is easy to fix, if people wish to do so, because there is lots of women's art out there.
If you're an artist, put women in your work, especially if you are female yourself. Promote the work of women artists. Got a show at a gallery or museum? Check their content to see what the balance is. Favor those with diversity.
If you're a teacher, use examples of art from all genders, and not just the standard canon that everyone is tired of anyway. Go grab some current stuff to illustrate your points. I've done this for a class at least once, using art by people I know as well as others, so at least one each of male and female in the mix.
If you're an art collector, even on a small scale, go count your content. What's the gender balance? (I think mine actually leans female. Hmm, I need to watch for some two-spirit art though.) If it's not close to half-and-half, consider seeking out women's art the next time you go shopping. If you can't find any, bitch at the proprietors for being sexist. Then tell them you're going to shop online, where there are lots of women selling their art. Send them an image of what you bought and rave about how it is better than what they had in their sexist art store.
If you're a gallery owner or museum curator, go count your content. Is it roughly half female, give or take a few percent for other sex/gender identities? If not, it's sexist. Go fix that. If you have the power to fix it and you don't, you're an asshole and why we can't have nice things.
If you like museums and galleries, you can help. Count the content. If it's not roughly half female, nag the fuck out of people in charge. Make a stink. File a complaint form. Did you pay money to get in? If you did, demand a refund because you thought you were going to see a decent display of art, not a sexist show. You probably won't get your money back but you will embarrass someone. Tell them you are about to pan them on social media and then send them a copy of your slam review afterwards. Detail your count and encourage readers to boycott the place until it stops discriminating against women. Hunt down places that offer equal representation, or specialize in women's art, and review those too, encouraging people to visit them. Send the sexists your glowing reviews of their competitors. Is it a publically funded museum? Many are. If so, complain to the funding organization, or a civil rights group that monitors the government, about giving money to bigots. Ask to send the money to inclusive institutions instead. Then tell the offending institution what you did. Punish them for being a bunch of sexist pigs.
Regarding themes: some shows and institutions have a specific focus. If they have more than one, there should be a balance between featured men and women artists. If it's monofocal, you're pretty much left to pester people to launch more institutions focused on the work of women, since stamping out thematic art exhibits isn't a great idea. Lobby the funding organizations to cease funding new ones that only feature men until after equity has been achieved.
The people in charge may not give a flying fuck about women, but they certainly care about their reputation and their bottom line. Reputation is everything in the art world, where value is created entirely by popularity. They are deathly afraid of being seen as unfashionable, undesirable ... so last century.
You can do the same thing for ethnic diversity, socioeconomic status, or whatever else you care about.