The story of Joan and Seth is not as uncommon as one might think.
The husband, busy with his affair, forgets about his wife. His wife, not wanting to be home alone, begins to create a social life.
Into the mix comes possibilities, and in many cases, there is no reason why she should turn them down.
Seth has already torn up the marriage vows. He is unfaithful. Why should Joan remain chaste?
The joke, if there is one, is that the husband, the cheating husband, will never suspect his wife of having an affair.
Firstly, whatever is the stated reason why she is out; such as attending the meetings of a particular club is true. She is at the meeting every Wednesday.
That she spent the hours pre meeting with Syl is not obvious. For unlike the cheating husband, the cheating wife does not want to be caught or even give a hint of infidelity.