Planning before Engagement

I think this is more a feature of people's definition of engagement than anything else. In the old days, people considered themselves engaged when they agreed to get married. Planning before that would be silly, because why plan for an event that may or may not ever happen? But if you define engagement as meaning some big formal proposal and a ring, and you know long before that that you're getting married, why not plan once you know?

The furthest we've come to pre-planning was hypothetically picking bridal party members.

Yep, that was pretty much my feeling. When I proposed, we had never actually talked about getting married. (It had only become legally possible three days before.) I wasn't sure she'd say yes. (I knew she wanted to be with me, but marriage was legally complicated.) So my proposal was a nervous, "So what would you think of the idea of getting married?" There was no planning before that, and both of us knew at that point that we were engaged (even though we never did have engagement rings).