It looks like Anna ( like most women) is rewriting history in her head as far as that office party goes. Is it possible she took more shots than she's letting on, Hank noticed she was drunk as a suck, ushered her off to side office down the corridor and took advantage seducing her and knocked her up. After the party Anna experience with Hank made her horny and as soon as she arrived home, in order to cover her tracks, she has sex with her husband to make it look like he is the father. Remember, she has a strong sexual desire for older men