I think one of my hang ups, and the reason I often bounce off setting change AUs, is that I really feel like circumstances shape behaviour, and if I know more about the place and time, including details, but often more the feel of it, or what people like that thought, then it feels more real to me. I always like the Ursula Le Guin quote:

As for “Write what you know,” I was regularly told this as a beginner. I think it’s a very good rule and have always obeyed it. I write about imaginary countries, alien societies on other planets, dragons, wizards, the Napa Valley in 22002. I know these things. I know them better than anybody else possibly could, so it’s my duty to testify about them.

Which could be more self serious than it is, if you don't consider it's Le Guin.

Anyway. There's only one bed. They fuck. It'll work out.