I think Izzy is a "comfort" gal. She bakes for comfort and she cooks comfort food when she is sad or in need and she turns to men for comfort, Denny was comfortable and George is comfortable to her. Not like Meredith who seeks out men/sex to fill a man approval daddy void. Izzie knows she is pretty and is secure in herself as a woman but she seeks the comfort things in life. She is also a comforter so she always crosses the line in comforting patients emotionaly as well. She is such a tactile person, food, touch, everything all soft and comfy in her world.
George and Izzie have been in a relationship for a long time now as friends so they are comfortable with eachother so the sex is strange but also comforting in a way and George as he has said doesn't want Izzie not to be there for him because he loves her and he loves Callie. George and Izzie share the recent death of loved ones so again the comfort thing popped up. Izzie, I think, is mistaking the comfort of George for it possibly being something more like I mistake chocolate for nutrition sometimes. George didn't need to say anything in the closet because the touching her hand, hitting the shelves says it all, he loves her and is mad he/they crossed the line and that he crossed Callie but he doesn't want to hurt Izzie by being mad and so he says nothing but I think she gets it.
I just so hope that Callie doesn't get hurt. I like her so much. I saw her the other day on You've Got Mail movie and she was the girl who doesn't want to run Meg Ryans credit card in the deli store and Tom Hanks gets her to do it. I almost didn't recognize her and had to look at the credits to be sure. Wow didn't realize she had been around that long.