Because its OK for blokes to play away... apparently.. while Imogen got all the flack, Giggs (Even though he is a legend) got away relatively scott free.
Another example is Kristen Stewart, look at all the flack she got for having an affair, while the bloke she had the affair with was barely mentioned.
No it isn't. They're as bad as each other and both to blame for the affair. The reason people give Imogen a hard time is that she's basically famous for having an affair with a married man and making a career off the back of it. Not something that she should be proud of or that the media should be rewarding.