The official WikiTree position as described on the page Michael links seems to be that we can leave the Sex at Birth selection blank and check the "other/do not display" status. This may or may not address Jessica's point.
However, Cathy, it sounds as though you want to set the Sex at Birth on a particular profile to Female. This depends on what browser you are using. But on mine there is a big arrow just next to the field. If I click that, three selectable choices appear: blank, male, and female. Are you not seeing something like that?