That's how I read that scene, with the letter. It wasn't that she doesn't like Tony, it's just that so many thing he gets to have, no one else does. And it always seems to cost her. No wonder she was pissed.

Not sure I buy the boy talk, nor it turning Maria and Pepper against each other. Like either of them would be remotely surprised that Tony Goddamn Stark slept with anybody. Maria wouldn't care, and Pepper's known him for too long.