The best advice I can give you SG is this :

Almost everyone new at this tries to do the following :

a. Argue
b. Negotiate
c. Beg

This stuff doesn't work.

You don't want to be a jerk about this, but you DO need to do the following :

a. Lead the meeting
b. Be the first to end the meeting, BEFORE things get ugly
c. State your position and IGNORE the negative responses and naysaying...

You will likely hear lines like :

I need space
I need time
I can't think right now
He's just a friend

Or if she admits the affair :

We are in love
We are soul mates
We understand each other
We are getting married
This is fate, it is meant to be

STuff like that

You will also hear :

I want a divorce
We shoudln't have gotten married
Our marriage is over
We are done
Our marriage was a mistake
We aren't right for each other
You are too controlling

All of this is garbage SG... and you need to just KEEP STICKING to your script when you hear all this crap...

Important points to get across :

1. Our marriage needs time to resolve
2. You are contaminating this marriage with an interloping third party - you are CHEATING me and YOU and our MARRIAGE
3. It must stop NOW - No more contact from OM
4. I will not lie to cover up your affair