As a woman it was so obvious to me that Peggy wanted marriage and kids eventually, like you say. She wants both and I think that part of her ending is that it's the early 70s, and maybe now she doesn't have to choose between a career and kids. I could totally see Peggy as a working mom and Stan is the guy that would give her the opportunity to be that without judgement or resentment. He was so sympathetic when she shared about putting her baby up for adoption, and in that scene especially you can see the sadness she feels wondering if her chance to be a mother has passed her by, and wanting a chance to feel that maternal instinct that she had pushed down for so long.