I JUST DON'T GET THIS WAY OF THINKING!
After some silly post in the stealing food thread my hackles are raised. Seriously, how much of a knob are you to expect someone to calculate the miniscule amount of extra electricity or water (WATER?!! I MEAN COME ON GUYS) they might use when staying over.
If, IF, your flatmate never stays over at their house and the b/gf is ALWAYS staying at your house then there might be cause for irritation. However, if, as I would assume most do, your flatmate stays around their b/gf house plenty of times too THEN QUIT YOUR WHINGING. The miniscule amounts of extra energy used are balanced out you momos so just deal with it and stop being such a tight numskull.