Proper way to issue an ultimatum?

[amg2]

(Cross posting)
I have changed the name of my thread as I'm not sure the old name was any longer appropriate. I hope that's allowed here.

Things w H have been stable, texting often, me still doing more than him, him not in counseling, no R talk. Went on a social outing last weekend that went well enough.

Over the weekend he had Yahoo instant messaging (IMing) start on his cell. I didn't say anything b/c he doesn't know what I can or can't see, if it just looks like simple "data" or "internet" use, etc. Last night he was IMing from 10:30 pm to 1 am and then again starting at 5:30 in the morning and going on most of the morning. I decided no reason to wait to ask him about it. He had been texting me much more than usual this morning. I said "can I ask who you were IMing with at 1 this morning?". He is a teacher and said that he had set up a Yahoo group to IM with his students about an upcoming AP test rather than texting them one by one. I said "at 1 am?". He said he had been sick all weekend and had his sleep schedule messed up and wasnt tired. But there were messages coming in AND out. He says he was "testing" it by creting two email accounts to see how it would work/look to the students.

It is POSSIBLE that all of this is true, knowing him and his teaching style, etc. It is also POSSIBLE that this is BS knowing his history. I didn't say too much but then asked if he would send me a login to all of this so I could see it like he and the students can and it would verify his story. At first he said "I think so". When I put a time limit on when I expected to receive this (so he couldn't go recreate the whole thing) he backed off and said "he wasn't in a position to prove himself to me." I said "so are you saying that you didn't do anything wrong, but that as a matter of principle you aren't willing to prove it?". He said yes. I said well you're not comfortable with your situation and I'm not comfortable with this either, so I guess there's nothing more to talk about. Goodbye and hung up.

I think he's probably lying but seriously w/ his teaching and these advanced students I could be wrong. I don't think it matters whether he's lying or not, b/c I can't continue to be open to him, talk to him, have outings with him, if he can't ease my fears in this type of situation. Just last week he texted me to tell me that one of his EA texting partners had contacted him. He was in a position to prove himself then.

The call was left very open ended which I'm glad for. I understand that my asking for a login seemed controlling to him, but I still think it was reasonable and I wasn't being reactive. It was genuinely something I wasn't willing to continue on without. I know he's going to contact me this week about items he needs from the house for the weekend. I think I HAVE to go dark at this point. I'm also thinking of sending a messsage along the lines of what Puppy suggested a while back on my cross post. Something like:

"Regarding our conversation today, I just felt you should know that in order to feel safe, considering your repeated affairs, I needed to know that you were not having inappropriate contact. As we've discussed, I 'want that out of my life'. Do what you feel you must do, and I will do the same. While I had hoped to avoid a divorce, my patience is not without limits."

The phrase "I want this out of my life" is something we both find ourselves saying about the current infidelity/separation situation.

ANY ADVICE APPRECIATED. If I'm going to say something before going dark I need to do it before he contacts me about items he needs from house. THANK YOU