If a guy says he has a very active sex life with his wife but he is...

Not necessarily . It might be active, but not satisfying .

Then again, he'd have to be a fool to say his sex with his wife is active when he's cheating on her .

About his active sex life with his wife? Well, it depends:

A) Assume he is lying about actively sexing the wife. Meaning the truth is no sex with wifey. That's a common "reason" to cheat. But its also a bruising admission to his ego. So if he's a cheater who gets no sex at home, of course he might be lying about the busy home sex life. He's already a proven liar (in order to cheat) right? AND, since he ties his "happiness" or "worth" to sticking his dick into a woman (otherwise he wouldn't cheat, right? ) then he would be prone deceive to avoid perception as sadder/more pathetic because his wife has rejected him.

B) Assume he is truthful about boning the wife all the time. Of course he could still cheat. Maybe he's just a lying, unfaithful horndog that screws everyone he can. Cheating don't make you impotent with the wife.

Not necessarily. He has to 'keep up appearances' so to speak, so if he is cheating he may be getting sex from both.

He may have a very active sex life with his wife and still cheat on her. In my experance when a guy cheats it may not be a lack of sex that is causing him to cheat but a lack of quality that is causing him to cheat.

No, he is just not in love with his wife.