It sounds to me ...from the time line and actions that your husband could be trying to use this to cover his tracks as a cheater with an all about me attitude. Post stuff on line get a GF then read the books then have you read the books ...then disclose ..." gosh I'm glad you agree because I already have a girlfriend I've been seeing for X amount of time."
HE DIDN'T FORGET THE BOUNDARIES. Pretty big boundary to forget..... He manipulated the situation to get what he wanted. Steamrolled. Are you the type who rather not want to make a scene especially around unfamiliar people?
Did he make the pronouncement of the physical intimacy during dinner in front of her?
Do you look forward to finding your own outside partner?
Have you told your children of this lifestyle change?