Q&A What Kind of Affair Was it? Does it Matter?

Hi Rick -- We've just finished EMSO and are nine months from D-day. My husband confessed to a 15-month-long "sex only" hookup with a woman he met on a dating website. They met 6 times before he convinced her to have sex with him. SIX DATES! From there, they had sex every 2 weeks--about 20 times total. He ended it about 6 months before D-Day, because we moved to another state for his work. 

I insisted that he find her real email address (not just the encryped email address of the dating website, where he had deleted his profile) and send a "close the door" email to her. As soon as she got HIS personal email address, she sent a couple of coquettish emails to him full of personal jokes, written at 2:30am. Next came the "close the door" email; she replied "message received." 

Was this an affair? He does not tell me details, so I won't develop intrusive thoughts. After woman #1, he then joined "Tinder" and hooked up with two more women until discovery. I feel that he's lying, when he says he had no relationship with woman #1. The lying bothers me and makes me think he's hiding other things. Do I need to know where they met and what they talked about so I can get full disclosure? Do we need a polygraph? We are both starting 12 step programs (CODA for me). He has a drinking problem, that is usually under control, but not always. 
Thanks for your thoughts!