Shag her. If the sex is great shag her a few times more.
In a few months come back and create the "I screwed around and furked my marriage up" thread.
Alternatively in a few months come back and create the "I am now married to the lady in the office" thread.
Life is full of endless possibilities. Any choice you make today determines future outcomes. Positive or negative is not known till you just about take your last breath.
The basis of such a relationship will be deceit and the cornerstones miss-trust. No trust = problem.