Janine has become comfortable just being with her daughter and feels her husband is intruding, is to "male" too noisy, he is invading her new turf. I experienced a similar feeling when my husband is out of town and it's just me and my daughters; when he comes home he is messier than us, wants to set new rules and rhythms while we have become comfortable in a girly groove.
I think her affair is about boredom, looking for the thrill of the unknown, and I think that the affair is her way of getting out of her stale marriage. She knows what she is doing doesn't make sense, i don't think she wants to marry this guy, but she is willing to, even wants to get caught and either get her husband's attention or get out of her marriage.