Iron Man's Tony Stark identity is public knowledge, so MJ knew who he was. He was only aware of MJ as a supermodel. That, and the fact that she jumped in, at great risk to herself, to save both him and Victor Von Doom (Who wasn't wearing his usual armor and fights alongside Iron Man for the greater good). Later on, as he's already going out with someone, Tony invites MJ to work for him and it's clearly a straightforward business proposition. MJ accepts and that's the end of the second issue in the story. She never gets around to telling him about going out with and marrying (I'm still not aware of how and when they broke up) Peter Parker/Spider-Man.