Having others use your outhouse still involves their placing their own stinky butts on the same seat that you use, so choosing to also accommodeate them is
not really all that different. If doing so
on a regular basis seems too gross and icky to you, however, perhaps you could
compromise with your house-rule to allow for weather temperatures, and say that you prefer that guests "take it outdoors" unless it's really chilly at the time.