Have been reading all this, and, having been in the same position as you twice before now, must offer you some advice that I didn't want to take at the time, but worked for me, and made me feel better in time.
A person (male or female) who feels the need for anothers company when they are aready in a relationship MUST feel there is something missing in that relationship for them, or they would not seek out the company of another.
Regardless of who started what, it takes TWO to tango, two to make, and two to break a relationship, and in your case, the man has a relationship too, so he KNEW what was going on, and was prepared to cheat on HIS partner/wife, therefore he wanted to make a relationship with your lady knowing she had a partner. He didn't care, and neither did she about hurting their respective relationships.
This means you are better off out of it, because she had done it before.
As for "wev'e only kissed...." it is so unlikely its not worth considering, because a man or woman doing this KNOWINGLY is doing it for kicks they can't/don't get at home, and the thrill of getting caught is the motivation.
Leave, friend, go back to your home, and heal. find someone deserving of your attention, this lady plainly is not.
Take comfort in the fact others care about you and how you feel, and are willing to offer you the advice and shoulder to cry on, albeit metaphorically, and stop your son doing something that will affect him in later life, and only come back to haunt him possibly. Violence does not make anything right, and the other man will be looking over his shoulder for a time expecting a beating.
Rise above it, walk away with head held high, for you did not do anything disrespectful, and others will respect you for it.
Best Wishes,
Mike.