Dorothy tells Steve and Duke that the Hibiscus Country Club is for married couples only, but she is single and already has a membership and private cottage.
This is perhaps because she is also an employee?
Or, saying it was for married couples, could have been a lie she told as an attempt to get them to think of some place else to hide. It being a lie seems more plausible due to the fact that the guard asked if she was bringing in a guest.
It is also possible that the club had a rule that single men couldn't join, but single women could. This would prevent the club from attracting a lot of peeping toms, which would change the tone of the club and would discourage many women for joining. It would no longer be an "innocent" nudist camp.
