I think this is a little silly...how everyone's accusing Meredith of being such a bad person.
On any other show when something like this happens, it's because the character has "needs." This has always irked me because it's a cheap answer to why someone would use another person for sex.
But the moment there's any emotions put into the equation, Meredith's a little mean tramp. Do you really think George think Meredith's in love with him? He lives with her, he sees what "McDreamy" does to her. If he gets hurt, it's partly his naivety to blame. Hours earlier in that day, Meredith was talking about how she never loses interest in sex, in "the beast." Obviously she hasn't been getting any since it appears her casual sex phase has passed.
So, she had an opportunity. Doesn't she have "needs" too guys?