Should I see it as a good sign that he is going to see a therapist, or not?
Depends on the therapist. Sadly, not all are really very pro-marriage. Do you know if this one is? Do they have any specific experience with infidelity?
"Puppy's Short Version, All-Inclusive Advice" for Waylayed Spouses Who Just Got Bombed"
1. Get proof (of whether or not there's OM/OW). Hint: There almost always is.
2. Do either:
2a. Aggressive affair-busting (see Allen A's posts over in Infidelity)
Those are the only two things I've seen work. If #1 turns up nothing, then proceed to #2b. And in the meantime, GAL your ass off, and become the better option. Best case, you'll attract them back, and at a minimum you'll become a better person that will be more attractive to SOMEBODY in your next relationship.
