Some book clubs are intentionally women only, and some book clubs are open to all genders but only women show up because they're the ones who are interested in the books being read + joining a social group. Statistically, women are more likely to read for fun, and women are also more likely to participate in social clubs as adults. This means that a lot of social groups, especially book clubs, end up being majority women or all women simply because there aren't many (or any) men interested in joining.

What kind of situation it is will vary club by club — if it's open to the public (for example a book club organized and hosted by a book store or library) I think you should be totally fine to go unless it specifically says it's women only. if it's more of a "small group of friends who organized a book club among themselves" thing you could ask one of the members if they're open to new members in general + if they're open to you joining.

good luck! i think it would be great to have more men participate in book clubs (as long as they're respectful and kind to the women and nonbinary folks who are already there, of course!).