I understand how your little affair was "just a mistake." Let me help you understand it, too.
Of course, your little dalliance with your pet slut was "just" a mistake, but let me help you understand the consequences and ramifications of your mistake. In education, we say a "mistake" creates an opportunity for new learning, so we will treat this little discussion as a "teachable moment." You need not take notes, but you probably should expect a test on this material.
First, sweetheart, you cheated on ME with her. I objectively have evaluated her assets, and I don't see a whole lot there. She is not nearly as cute as I, so you miscalculated there. Not by any means will she ever match my intelligence, intuition, and insight, so you failed properly to judge the depth there. I concede that her boobs are bigger than mine; but most of her outweighs, out-measures, and super-sizes me. You may have miscalculated the safety hazard, sweetie, because I fear she could have crushed you; did you wear your personal protective equipment? She hasn't mothered any of your children yet, has she? So you may have missed a value judgment there. I'm sorry, darling, but your math just doesn't seem to add-up.
Still, it's just a mistake.
Second, we probably should take a long, hard, analytic look at what this "just a mistake" says about your respect and esteem for me.
Oh, no, I know you certainly never meant it that way; you never meant it that way, because you never bothered to think about it. That oversight might have been a mistake, don't you think? I feel very fortunate that I know my own value in this world and out there on the singles market. I don't need to test it. I wonder what made you feel compelled to shop around? What made you think you so totally and completely could disrespect me?
What made you take me so completely for granted? And, really the most insulting among the many insults: what ever made you think I was so stupid or so oblivious I wouldn't notice or wouldn't care? Even your dog knows you've been cheating, and he feels disrespected and disappointed, too.
Still, I absolutely understand your affair was "just a mistake."
I'm sure that, just as I did, your teen-age daughter, who used to worship you, will understand all those claims about "working late" weren't really lies. They were simple factual errors. You simply confused your geography, mistaking her house for your office; and you simply confused your kinesiology facts, mistaking sex for work. Although from where I stand, I can see how sex with her might have been quite a bit of hard work, still you did make a factual mistake.
Oh, and you may have committed just a little psychological error: do you think that maybe, just maybe, you mistook sex for love and affection? Or maybe, just maybe, when you cherished sloppy sex more than marriage and family, you might have put those things in the wrong order?
Yet, for all my concerns, I recognize how you made "just a mistake."
You'll pardon me if I worry that you may compound your mistake as you try to correct it. You won't blunder into the mistaken conviction that you can erase your mistake with expensive gifts, will you? You would live so to regret that error. And you won't try to slough-off any of the blame for your mistake on me, will you?
Surely, you recognize that I have loved, honored, and cherished you just as I promised I would. I fervently hope you will not mistakenly believe that your "just a mistake" will all just blow over or that we all might just pretend none of this ever happened. You don't believe that nonsense, do you? You see, honey, you will need a lot of therapy as you get over this tendency to make mistakes; and we, as partners, will need a great deal of counseling as we learn to deal with the consequences of your mistake. I would love to believe that something in your childhood inspired this huge mistake; or maybe something in your biochemistry went out-of-whack, so that you may plead some kind of "diminished capacity." In all seriousness, though, I think those two theories probably are mistaken.
Given all of this very serious subject matter, I know you will understand how I cannot risk the mistake of allowing you to continue living here. I absolutely understand and forgive your "just a mistake," but I really have no desire to watch you repeat it. That would suggest that your cheating was okay with me, and that suggestion would be quite a bit more than just a mistake.