Boundaries are absolutely necessary. Absolutely. It's in the HOW and WHEN these boundaries are enacted, that is extremely important.
I would say that Coach's last post is correct, but may be a little out of order.
Asking for what you want is important. Knowing what you want must come before that (your goals). Be smart in HOW you ask for what you want.
Delivering an ultimatum, as described on 230-231, is not the first step. But an ultimatum is not the first step in boundary setting.
Start with knowing what you really want in this relationship. Ask for it in a way that your spouse can receive it. If you demand sex right now, you're not going to get it. If you demand she stop her affair right now, you not likely to get it.
Making everything either do this or everything is over is not the best way to build a relationship.
Besides knowing what YOU want, it's important to know what SHE wants, that's where negotiation of boundaries comes in(and boundaries are usually negotiated).