What should I do about the marriage?

I have been married for 9 years 7 which were happy, but for the last two years and a few months my wife and I have stopped being a couple. Let me explain we still live together we have two sons one is special needs (he is high functional) and the other is in good condition. The problem is that ever since another man from the past showed up a the end of 2007 she has not been the same. That man is no longer in the picture as he "was not serious" about the relationship but now she is just sleeping around. It may sound lame but I do have unconditional love for my wife and I know that I have been, for lack of a better word an "enabler". Her friends have seen who she really is now and some have stopped talking to her while a couple still talk to her and ask her why she does what she does to me knowing the kind of man I am! What kind of man am I you may be wondering? I do not hold grudges, I help others when in need, My family comes first and her family knows this as well. Both her and I were in broken families as our parents did not stay together, the difference is that my family was close and I had support my wife on the other hand left her mom when she was 13 to live with her Fahter who although a decent man was not there as a supportive Father figure. When he caught her in bed with a boy in his house he did not scold her or talk to her about how she does not need to look for affection that way, no he told the boy to go Home and not comeback, and other things that he as a father should have never missed out on. Well that is in the past but I wonder now are these acts of a desperate woman who does not believe she can truely be loved? Now let me say that I am no saint either because for a couple of years gaming was something I did to get away from things, the problem is I think I got away too much. This in fact may have triggered something in her that now she does not know or care how to stop.
From what her friends have told me the men she talks to (I use that term loosely) do not treat her with respect as they only use her. It hurts me to know that and that she maybe believes that she is the one benefitting. Our oldest son has asked her on several ocassions why she is the way she is now.......he does not know what she does but he knows she is not as loving as before. I think maybe its time for me to move on and concentrate on our sons.......Its just that I believe that who I married is still there and she does not know how to comeback.