BTS, like many K-pop groups, follows a set of strict rules and guidelines set by their management company. Some of these rules may include restrictions on dating, alcohol consumption, and public behavior. These rules are often put in place to maintain the group's image and appeal to their fan base.
However, there is no specific rule that would prevent a girl from joining a K-pop group in general. K-pop groups have been known to have both male and female members, so it's not entirely impossible for a girl to join a group like BTS in the future.