Thank you for such a glowing review! I was actually contemplating for a long time whether I should put Smut tag or not for my story. In the end I've decided that Adult+R-18+Warning should be sufficient for the new readers.
And the decision was for that exact reason - these scenes also drive the plot. Some of that is obvious and immediate, while some parts of these scenes only hint at the future events.
Tell me if you think is should re-tag it.