Having a ritual or not is a side issue. It is neither necessary or sufficient to qualify a belief system as a religion.
Religion surely is a "sin" to rationalist - Maybe not a cardinal, deadly type sin. It's a lesser sin equivalent to what Catholics think about masturbation.
Ritual is one of the defining aspects of religion, actually. In my studies and meditations, I've determined that religion (that is, the concept that we generally think of and reference with the word) is primarily made up of three things: ritual, mythology, and philosophy. They all work together, influencing and deriving from each other, and exist on a cultural (rather than an individual) level.
I don't think any of that is inherently contrary to rationalism.
I think many types of religion (such as Abrahamic ones) can be regarded as a sort of "sin" to rationalists, and that some of these rationalists might regard those types of religion as the only type, but I'd chalk that up to faulty knowledge or understanding (perhaps deriving from modern anti-religious propaganda about what religion is and its relationship to humanity and history).