Is it normal that my sex drive is as high as ever but not for my wife?

I've been married since 2006. Always had a really high sex drive. My wife has always been hard work in bed - just doesn't know where to put her body, it frustrates the hell out of me. She knows all this but seems incapable of improvement. Every time we do it she does the same things, and I don't feel like giving her a lesson every time I get an erection.

Since having kids she's let herself go too. Her body just does not appeal to me. She's enormous.

Is is normal for a married man to want a loveless affair to just get some hot action? I don't want to hurt my wife, I don't want to leave, I don't want to lose my kids. I just want to f*ck the hell out of some hot woman who knows what she's doing.

I don't think I should feel bad for being dissatisfied and unfulfilled - have others been in this situation? What did you do? How did it work out?