Thanks for bringing your question to JustAnswer.
No, it isn't normal to be in love with your wife and still be looking for other people to sleep with. If you've never been able to be faithful, I have two recommendations.
First, go to this site and take the sex addiction quiz.
Then, find a therapist--male, if you'd be more comfortable--and work on the issues that make you stay on the prowl constantly. Bring the results of the sex addiction quiz to the visit. It's impossible to build an honest bond with your wife while you are so focused on only one part of a relationship.
Masturbating behind her back is actually a healthy way of releasing sexual tension--the comments above pertain only to your statement "if I have an opportunity to sleep with someone I like, I will and it happens a lot" --which jeopardizes your marriage for a quick physical rush. Sooner or later one of those women will find a way to tell your wife...or your wife will find out another way. Imagine how you would feel to find out that your wife has the same attitude you do toward sex--if she just did it with other people if she felt like it. The very thought of it probably makes your blood boil.
To give you marriage the best possible chance of survival, here's a basic rule of thumb: don't do anything that you wouldn't want your wife to do.
Get into therapy and figure out where you learned that scoring sex was more important than a loving relationship...that 's the first step toward keeping your sex drive focused on your wife.