Two points that were distracting regarding Ellen: 1. The child Ellen is portrayed as very alert and engaged, she would NOT have left school with a stranger. 2. NO school would allow a child to leave with someone the school did not know AND without express written signed permission from a parent to allow that person to take their child.
Dan had the responsibility (as a married man) to NOT engage in an affair and jeopardize his marriage and the protection of his child. That's why marriage vows state "forsaking all others". Yes, although in love with your spouse, there are in fact very attractive, interesting people out there that are NOW off limits PERIOD. Grownups get that. Having said that, initially Alex presents herself as being sophisticated, astute, and liberal. Initially she is NOT saying what's in this for me? As she does later on. Initially she is NOT saying to Dan you are only thinking about yourself -- what's in this for me? As a mature adult who should have known herself -- she needed to ask herself from the beginning, What's in this for me?