I get the same results both ways, even for days with no guests of one gender or no guests at all.

CREATE TABLE #Guests (checkin date, checkout date, FullName varchar(50), Gender char(1))

INSERT INTO #Guests

SELECT '10/12/2012', '10/16/2012', 'Corky Doe','M' UNION ALL

SELECT '12/12/2012', '12/17/2012', 'Janice Doe','F' UNION ALL

SELECT '11/12/2012', '11/24/2012', 'Howard Stern','M' UNION ALL

SELECT '12/12/2012', '12/13/2012', 'Abagail Johnson','F' UNION ALL

SELECT '12/12/2012', '12/15/2012', 'Teddy Sanft','M' UNION ALL

SELECT '12/12/2012', '12/18/2012', 'John Overton','M' UNION ALL

SELECT '12/12/2012', '12/19/2012', 'Sally Jenson','F' UNION ALL

SELECT '10/20/2012', '12/12/2012', 'Tiffany Blue','F'

DECLARE @StartOfMonth DATE = '2012-12-01';

WITH Calendar AS (

SELECT TOP (DAY(DATEADD(mm,DATEDIFF(mm,-1,@StartOfMonth),-1)))

[Day]=DATEADD(day, number - 1, @StartOfMonth)

FROM [master].dbo.spt_values Tally

WHERE [Type] = 'P' AND Number BETWEEN 1 AND 31)

SELECT [Day]

, Guests=ISNULL(COUNT(FullName), 0)

, MaleGuests=ISNULL(SUM(CASE Gender WHEN 'M' THEN 1 END), 0)

, FemaleGuests=ISNULL(SUM(CASE Gender WHEN 'F' THEN 1 END), 0)

, Guests2=COUNT(FullName)

, MaleGuests2=SUM(CASE Gender WHEN 'M' THEN 1 else 0 END)

, FemaleGuests2=SUM(CASE Gender WHEN 'F' THEN 1 else 0 END)

FROM Calendar

LEFT JOIN #Guests ON [Day] >= checkin AND [Day] < checkout

GROUP BY [Day]

DROP TABLE #Guests