In just a few years now, Amy has given up her apartment to live in sin with Sheldon in Penny’s old place, they got engaged after a proposition from another woman showed Sheldon just how much he needed and wanted to be with Amy, and then married not long after to great fanfare.
Once they did the deed things began to move pretty quickly compared to what came before.
Truth be told, if Sheldon hadn’t taken that big step on Amy’s birthday we might not have had as much growth in their relationship in the years since, or with Sheldon specifically.
Season 12 has seen them work on a scientific theory together and go through a lot of personal and professional trouble to get it completed, published and now they might be nominated for a Nobel Prize for their work.