Wow, a lot of stuff here. Whew.
Alex8:
One main thing you need to do is start being more understanding of what your wife is going through. If all she gets out of sex is pain, no wonder she doesn't want to do it. Put yourself in her place. What if every time you had sex, instead of an orgasm, your penis became very sore, sometimes to the point you had to go to the doctor? I don't think you'd be agreeing to sex any time soon. That's how it is for your wife. Stop being angry and resentful.
I think this whole thing has become a real sense of frustration for both of you (as well as other people here

). Being angry with her and thinking about cheating is only going to make the problem worse. I mean, if you are in that mindset when you do have sex, I'm sure it's not very sensual and loving.
A few questions: How long have you two been married? Was it always this way or did things change along the way? Maybe after she gave birth? That can be a cause of painful sex. You said she goes to the doctor if you guys have sex too much. What does the doctor do to relieve the pain? Does it tear things? Has the doctor suggested ways to make the problem better?
Does she have a problem with vaginal dryness? That could be why she gets so sore. Maybe using more lubricant would make things easier. Are you doing plenty of foreplay? If you are just jumping right into the act without a warm-up period, she's not going to be into it. Do you know where she likes to be touched and kissed? If not, you need to explore around. Ask her want she wants and what she likes. Sex is a two-way street. Tell her that you want her to enjoy herself, too. In my experience, there is nothing that makes a man feel more like a man than giving a woman pleasure.
Also, how long do you have sex when you do it? You should limit the actual penetration to no longer than 10 minutes or so. The vagina can only take so much. That's why you need the foreplay.
Caroline suggested dilators, which I think may be a viable option, if your wife is willing.
I know that people here have been debating the desirability of giving a hand-job, but I don't see why that's more "disgusting" than any other sex act. It's all in how you look at it. I know the penis isn't the most beautiful thing in the world, but neither is the vagina (no matter what the monologues say).
I think that if vaginal sex is painful, mutual masturbation may be a viable option. You can even do it to each other at the same time, although the coordination may take some practice. If she doesn't want to get her hands "dirty," wear a condom (yes, I know you probably think you shouldn't have to wear one with your wife, but how about compromising?).
If she has always been this way and is totally unwilling to work on the problem, is it possible that she has any history of abuse? I don't know your wife, so I'm just asking. Sometimes that can cause problems in this area.
I hope you two can work things out soon. Please write us back with your thoughts and let us know if we can be of further assistance. Good luck.