Earlier quoted context omitted.
ORM is decoupled in rails to my knowledge... there is a default to AR which is easy to overwrite with your ORM of choice (which often provides an ActiveModel interface and further rails integration by installing one extra gem). tl;dr: providing sane defaults != strong coupling
OK, thanks, my mistake. Not coupled. But still, there is a strong presumption that your app will use a database and wants a database.yml. The few times I've wanted to just start a Rails app with simple Erb, with models representing data concepts (but not necessarily SQL anything), and evolve out to using a database (if and when needed), have been frustrating. It's something that has been easy to do in some other fram…
If you're planning on using a DB eventually but don't want to deal with migrations etc. for now, then start with a NoSQL DB like MongoDB (you don't even need to describe your fields, it can be completely dynamic). You can always sub in an RDBMS later on if your design calls for it.