In the past there was obviously far more female nudity in movies.
However, this has changed now to such an extent that there is probably just as much or more male nudity.
I think the reason for this is that female nudity has come to be associated with juvenility and exploitation, and therefore is often avoided, while male nudity has none of these associations and in fact can do the exact opposite, making a movie seem less juvenile and less like exploitation.
Not only do directors avoid female nudity for this reason, but the actresses themselves likely wish to avoid being associated with these things and are more reluctant to do nude scenes. Males on the other hand only have their reputation improve after doing a nude scene. It’s a strange double standard.
Also, males show more revealing nudity than women. Most female nudity in movies is restricted to breasts. I do not find compelling the argument that a female showing her breasts is equivalent to a male showing his buttocks. This argument makes no sense - you are basically admitting that women show less nudity, and then saying that their nudity matters more or something. Which may be true, but is not the point being debated. Also, you will see a penis in a movie far, far more often than female genitals.
Males show more nudity, but female nudity is assumed to matter more, and this is mistaken for them showing more. Even references to female nudity are actively avoided, as in the Batman example. Personally, I wish there was more female nudity in the style of male nudity. Males often show nudity in good natured comedies, or serious sci fi films (The Full Monty and 12 Monkeys are good examples). Females are unlikely to be nude in films like these, and are more often seen nude in juvenile movies or exploitation films. And, of course, that is largely why female nudity is associated with such things, and why it is avoided in the kind of movies I like to see. Oh well.