You can use an expression in each of your Gender Percentage
Female/Male columns to get the count of each gender in that group compared to the number of members in the group:
Male
=Sum(IIf(Fields!Gender.Value = "M", 1, 0)) / Count(Fields!Members.Value)
Female
=Sum(IIf(Fields!Gender.Value = "F", 1, 0)) / Count(Fields!Members.Value)
This is getting the total male or female members in the scope then getting the percentage of the overall member count.