Yes Grace, and it's pretty evident that for our OP, the situation is still very painful - and so at the moment, she is still working through that resentment - light years away probably from loving the people who she might believe destroyed her life. As always, the best revenge in life is to live well - but it is often only from a distanced position, when life is better and one is happier again, that it is possible to make that choice and live without resentment.
I'm afraid I don't agree that people never go into affairs lightly - if only that were true. I think people are in huge denial about the pain and the devastation they cause - and the ripple effect on so many people's lives, including their own. I've known many people kid themselves that this is "just a bit of escapist fun" - and others who deny up to the point they have actually had sex, that they are even engaging in an affair. They went into affairs very lightly - always underestimating the damage.
And yes, it happens - but it is still wrong. It is a choice, like any other in life.
I wrote recently on a thread about those people who we perhaps all know who are in very unhappy relationships, who don't love their partners. I have a friend in just that position - and she has had several offers from other men. She has very firm views that she is staying in her marriage by choice - a choice that others may not agree with, but one exercised with free will. She has said that she would never be unfaithful, despite the offers presented. In her view, infidelity is wrong and she believes that until she makes a different choice, she will not engage in something she fundamentally believes to be wrong.
One of the biggest lightbulb moments for me, having been fed a diet for most of my adult life that "no-one has an affair if their marriage is happy", is that infidelity is not an accurate barometer of a marriage. It happens in good marriages and like my friend's, it often doesn't happen in bad ones. The determining factor then, is the people in those marriages and their behaviour choices.
Brahms, not sure how old you were when your DF left your Mum, but I can offer my insights as the child of a marriage that ended very acrimoniously, but with no third parties involved. Both my parents went on to meet my much-loved step-parents and I never felt like you do, that I was "sharing" them - and my DCs only benefit from having "extra" grandparents. I would venture that you felt the way you did, precisely because infidelity brought your parents' marriage to an end - it just skews the picture every time. My in-laws' marriage ended after DH and I were married - after longstanding infidelity of one of the parties. This had a far greater ripple effect on DH and his siblings - even in adulthood - than my parents' marriage break-up had on me or mine.
Sayithowitis speaks from the heart - infidelity affects children long into adulthood - and I honestly don't think people realise quite how much, when they are crossing that line and engaging in an affair. Even as an adult, it takes a lot of work - and real honesty from one's partner, to convince oneself that it wasn't you, or even your marriage, that caused your partner to be unfaithful. That the problem was with them and not you.
After his affair, my DH and I made huge changes to our relationship, but the most significant behaviour changes have come from him - fortunately right from the start, he took responsibility for his behaviour and never once blamed me or our relationship. He realised pretty instantly that the problem was with him and went to therapy. It has made him almost evangelical now in his belief that it is all about personal responsibility - we can never blame others for what we do.
A child doesn't have those resources - and often never gets past the feeling that they were somehow not good enough, or loved enough, for their parent to stay. I just hope you are able to do that now, Sayithowitis - and although I don't know you, I'll bet that you were determined to ensure that your children never felt this way - and that you spoke openly with your DH about fidelity and the value you place on it.