It is an absurd agreement.
You want a commitment that if she gets a boyfriend and doesn't tell you, and if you find out later, then custody reverts to you. That makes no sense and doesn't address the best interests of the children. That agreement makes you look angry, vengeful, and controlling.
The affair is ancient history now. You have to get over it. The affair doesn't affect custody and time sharing issues. At all.
You can't realistically hold her to her promise to stop seeing the guy, and once your well and truly separated she can do whatever she wants with her personal life as long as she's doing a decent job as a mother.
This agreement, if signed, will obviously be done without any independent legal advice to her because no lawyer would tell her to sign it. Then again, maybe they would say "go ahead and sign it, because it's entirely unenforceable and no judge will change custody of the children to dad just because you got a boyfriend".
I'm trying not to be harsh, but you need to give your head a serious shake, here.
I'll await your reply. I know that there's a lot on your mind, and I'm not judging you or your situation. But you're nothing thinking straight.
If I've answered your question then I'd appreciate positive rating, but I expect that you'll reply which is perfectly fine.