Three quick points, while they are fresh in my mind:
1. When you discuss the MC thing, and Rob's script above (which i love), I would add only "I won't waste my time or our family's money . . . " Much more powerful, and reminds a cheating (or thinking-about-cheating) spouse that you will NOT tolerate them squandering the family's finances on their affair.
2. Re: "soiled panties or condoms in the glovebox." Lest you think this is hyperbole, I would point out to you that BOTH have happened on this very forum just within the past 30 days.
3. "Amen, Amen Amen" to Rob's (or was it McQueen's?) suggestion to PRACTICE WHAT YOU'RE GOING TO SAY. Role-play it in your head, or -- better yet -- in a secluded place, OUT LOUD. Practice what you're going to say, HOW you're going to say it, your EYE CONTACT. Anticipate what SHE will say, and how you are going to respond. I sell for a living (and am very effective at it, and am a very good public and personal speaker), and yet I felt I needed to do this, as it is in these stressful moments when we are easily at our WORST. And at these times, you need to be at your BEST. And might I add, "I nailed that sucka."