In RE update to and including version 7.96 patch 6 there were only every three choices for gender; male, female and unknown. What is more a record must have one of these values. In the constituent mapping area the gender defaults to "Unknown" just as it does when you create a new record in RE. This is fine for new records but you do not want the record to be overwritten with "Unknown" when there may well be a value there of male or female.
That is why by default from version 2.3 patch 5 of Importacular, a new constituent mapping will add a value of Unknown to a new constituent but will not update an existing record. This is done in the field settings as shown below (your field settings screen may look a little different if you have a different version from this one).
The Only Update Values For... Existing Constituent records is unchecked meaning that if the constituent already exists it will not update the gender value.
If your mapping does not have this set up then it will overwrite existing constituents with a gender of Unknown so you should ensure that this is set up in this way.
From RE7.96 patch 7 the gender can accept other values and no value at all. From this patch (and from version 3.0 of Importacular), the default value is blank.