If you're going to tell her you have to think about it, then why do you have to have all of the answers right now?
Your position needs to be "As much as I'd love to reconcile, I'm afraid it's more complicated than that now. I need time to think about this." Anything more than that and your wife will forever put you as Plan B in her mind, waiting on a string like a puppy dog (ok, so I just mixed metaphors there, lol) if her Plan A other man doesn't work out.
I would just LISTEN beyond that. What you're looking for is some sort of self-awareness and introspection on the part of your wife as to WHY she saw this (having an A) as a solution to the problems the two of you were having. Otherwise, even if you DO get back together, you will forever be wondering "OK, so if the marriage goes less than perfectly again in the future, I have to worry about her cheating on me?"
At some point, you can ask her "How might you have handled things differently?", but I DO NOT THINK THIS IS THAT TIME. Right now, the message you want to send is "Wow, I might have really screwed the pooch with Bart this time. He seems to really still love me, but he's also not running back to me like I thought he would."
I'd welcome others' opinions as well, as I know you would. Especially the women.
