Morning OP - I hope you are coping ok at work 
I'm baffled as to why your DH is acting like this? Does he really think a night in the spare room is all it takes?
Remember that you found out by accident, he didn't confess to you what had happened like a truely remorseful husband would have.
He previously kissed this woman and then the other night, together, they engineered a situation which meant they could be alone together. They both wanted and knew something was going to happen between them. If your DH was really ashamed about the first kiss he'd had with this woman then he would never have agreed to meet up with her that night when she asked him to, but he did.
Whatever happened the other night, be it another kiss or sex, was not a spur of the moment stupid mistake.
If you hadn't have seen his text messages would he have told you what had happened? Or would you still be in the dark whilst he and the other woman sneaked about behind your back? I may be wrong but I'm guessing the latter.
You deserve so much more.
Maybe I would have more sympathy if he'd broken down in tears when he had got home (without you having seen hid texts), told you that he'd done something really stupid, said he couldn't lie to you and that he was so ashamed of himself and so sorry, but he didn't.
He's just a cheat who got caught out and now expects you to just move on because it was 'nothing' and he's done his time in the spare room.
Why don't you try writing out an email to your DH (but don't send it) and write down exactly how you feel and how hurt and confused you are by what he has done. Print it out and when you get home sit him down and read it to him without letting him interrupt. Sometimes it's easier to read out how you feel then wait for the right words to come amongst tears and arguments.
If he understands just how much he has hurt you and what damage he has done to your trust and your marriage maybe then he will realise how it wasn't just 'nothing'.