Girlfriend VS Mistress

Good night hearters. I don't know if you are into a Tv Serie called "Mad Men", is about an american advertising company in the 50's and I've been watching it since the begining and my mind is been wondering abou the following...So the main character is married to a beautiful house wife, every men's dream, such a barbie doll, such a dedicated wife! She expresses her burning desire for him, she wants him so bad but she only gets sweet love, missionary position and lights turned off. Off course she is a sad young lady, with unsatisfied and hidden desires, not because she wants too! But then her husband screws around other women in the dirtiest passionate ways you can ever imagine.
My point being...Why is it that mistresses get all the fun? Aren't proper women allowed to be dirty and to enjoy sex? Shouldn't your girlfriend be the figure of your deepest fantasys? I know nowdays things are slightly better but after been having some girl's talk I realised that most of times dirty girls aren't taken serious. Just because a girl likes to enjoy herself and knows exactly what she wants in bed doesn't mean she had been with tons of guys or that she isn't worthy of trust.
Just get over it and fuck your girlfriend the way she deserves to. Then you can cuddle and fall asleep together with kisses and sweet talk ;)

R.