Retrieve latest update time for male and female members with low selectivity?

This is a SQL Server and want to find out the latest update time for male and female members. What is the most efficient way to create an index to find the data I need with such low selectivity? Thanks for all your help.

SELECT sex, COUNT(1) FROM membership

Produces:

And

SELECT sex, max(UpdateTime) FROM membership

Produces:

The first query can be supported purely by a single column index on (sex). But both queries can instead be supported by the following index

INDEX IX NONCLUSTERED (sex ASC, UpdateTime DESC)
-- alternatively
INDEX IX CLUSTERED (sex ASC, UpdateTime DESC)

The second query could be improved in this case, by instead doing separate TOP 1 queries for each value

SELECT
  m.*
FROM (VALUES
  ('M'),
  ('F')
) v(sex)
CROSS APPLY (
    SELECT TOP (1)
      m.sex,
      m.UpdateTime
    FROM membership m
    WHERE m.sex = v.sex
    ORDER BY UpdateTime DESC
) m;

If you have a column where you don't know the exact values, but wish to do a similar skip-scanning technique, you can use a recursive CTE and a function instead.

CREATE FUNCTION dbo.GetTopValue (@sexPrevious char(1))
RETURNS TABLE
AS RETURN
    SELECT TOP (1)
      sex,
      UpdateTime
    FROM membership
    WHERE sex > @sex
    ORDER BY UpdateTime DESC;

WITH cte AS (
    SELECT TOP (1)
      sex,
      UpdateTime
    FROM membership
    ORDER BY sex ASC, UpdateTime DESC

    UNION ALL

    SELECT
      t.sex,
      t.UpdateTime
    FROM cte
    CROSS APPLY dbo.GetTopValue(cte.sex) t
)
SELECT *
FROM cte;