I must disagree, and say the Duck/Peggy hook-up makes perfect sense. But each one has a different reason for the affair.

Duck's motivation is simple. He is a middle aged man after some young tail. He chooses a Peggy type over a Joan Type, because he can feel more in control and virile with the less attractive Peggy. He can feel like he is showing her the ways of amor. His line about giving her a go around like she has never had, tells me he thinks he is schooling the girl.

Of course Peggy has diferent motivations all together. To get some insight into Peggy, you have to drop back much earlier, when she tells her mother she is geting an apartment in Manhatten. Mom reveals she is affraid she will become "one of those girls", and Peggy quickly comes back with "I am one of those girls".

Peggy is not looking for a husband. She sees the affair with Duck as an exciting career intrigue. She is sleeping with the enemy, playing both sides of the fence.She is doing what the guys do. Fucking for fun and profit. She does what Sal was unwilling to do.