Split a class by extending it, with a condition

Sounds like you want single table inheritance:
Keep your existing Person class, with all the common code, and add a string column called 'type' to the table
Now create classes Man and Woman that inherit from Person.

Now whenever you do somethine like Man.find :all, or has_many :women etc... it will be scoped to Man or Woman as appropriate. You can still go Person.find :all to get everyone.

If you already have existing data, you'll need to populate the newly created type column with the name of the appropriate subclass of Person.