Well for me it depends. If you can still get out of bed and act as what if... that is a good start... I think women can tell if you can do it. I think she came on to you because of your brief showing of pulling back and she was temp checking... obviously I am just guessing with the little bit of detail I can read from your posts.
Probably best not to have sex.... but i am like you.... can be tough not too some times. So if you cave, at least act as detached as possible afterwards.... I mean right afterwards... no smiling, etc, you could say "thanks" but that is much as I would say, and not smile so to speak... maybe like a one night stand and you got to go.... (I am only saying this once you cave... and you are in the middle of it, then at least salvage the rest...)
That is my humble suggestion. Also, you may backslide, but then quickly keep the 180 etc going. And also, I think it has nothing to do with weather a OM is involved. Just b/c she has sex with you -means nothing regards to OM. The only time she will pull the crying or emotional talk is when she is trying to push you away. In this case she may have noticed you pulling away so she did this on purpose.....
H (me) 52, W 42 M 15 D14, S12 PA June and Sept 2016 Found out Sept, confronted Oct NC with OM since Oct, remorseful Dating since Nov