Karen,

For your daughters to have to see this is awful.

It's important that you be able to tell them the truth (an affair is wrong, adultery is wrong) without totally villanizing their father.

In other words, I think it's helpful to say, "We all love Dad, even though he's making some very wrong choices."

And if you haven't talked to you daughters about the moral import of your husband's decisions, now's the time. I don't think having them grow up in an "I'm OK, your OK".