Irishdad, the checking in was more to get reassurance that nobody plans to say anything to anybody, but I do see the ridiculousness.
That is ludicrous. It's an attempt to keep contact going.
If you'd robbed a bank with him, would you keep texting him to make sure he wasn't telling anyone? Creating more evidence?
In situations where someone "rescued" someone else - usually from an abusive relationship or violence or a horrible family - there tends to come a time when you outgrow your rescuer. He is still stable and loving and trust-worthy, but he becomes more of a parental figure, a safe place, rather than an exciting person. In some cases it's possible to manufacture a sexual relationship that evens things up, to realign things now that you're equals. In your case, it seems that DH probably doesn't miss the sexual side, and he's had these issues before. That may mean you've outgrown this.
It's still your choice to stay, or not. It's not fair to cheat on him, though. Cheating on him twice at a conference has to be the most unsubtle place, and you're making excuses already - you know it hurts, but your remorse is mixed up with other feelings... It's romanticising what you're doing. You are making it something exciting, illicit. The thrill is cancelling out the guilt for now. Maybe there isn't guilt, but you feel there should be.
You need to stop talking to OM. Whether you want to admit it or not, you have an attraction to him, you've trashed your boundaries and you're at least starting to develop feelings for him. If you can't stop all interactions, you need to make any necessary ones stoically business-like.
Whether you tell your husband is a dilemma for your moral concious. I'd want to know, if I was him, but equally you're then sharing your unhappiness and lessening your own burden, and adding to his. That's not especially fair. Consider that everyone sees everything on conferences, though, so this may well get back to him, even if it takes months.