I think in general that this hatred of Megan by many is rooted in typical sexism BUT also in the fact that she is not an "original" character. I don't think we're used to seeing TV portray human relationships as they really are, and thus mad Men is frustrating to many. But the truth is, since the show is about Don, Betty is not going to be in it as much. Once you divorce someone, they drift out of your life. When you meet someone new, they are going to be featured more. That's how it is.