I think it's funny that Betty indirectly caused this premature engagement. If she hadn't fired Carla, then Don never would have needed to take Megan on his business trip/vacation with the kids. He may have ended up in a relationship with her, but the time alone out of the office with his kids really speeded up the process.
And poor Sally, Bobby, and Gene, with no Carla in their lives one of the only stable people they know is gone. I've always felt a little sorry for Betty because I think I would have lost my mind if I had to be a housewife at that time. But when she fired Carla and spoke to her so rudely, it really turned me off.