Just go back to the EXACT plan, and the EXACT demeanor, that you were doing prior. When she asks you 'WFT?' just respond as suggested by someone above.
Other than legally, it's entirely fixable. Not sure how your state works, but some jurisdictions that are "fault" ones, that do allow an "infidelity" action, will see the LM as tacit approval and a wiping of the slate as far as the adultery is concerned.
Medically, if you didn't use protection, I'd strongly advise that you get tested -- now, and again in about six months.
