@Coin, I'm going to be briefly frustrating (on purpose, that is) to make a point: Science Fiction is, IMO, a sub-genre of Fiction.
This may be true, but so what? I hesitated early on to be clear that I am not the Genre Police because statements like "punk is not a musical genre" leads us down so many rat-holes to fill any ten academic institutions.
I once insulted an author at a dinner with authors after he said he's working on a Steampunk Novel and asked him, "What is Steampunk?"
I was once insulted by a panel of authors (known, published names at that) when they said that Cyberpunk is empty and challenged the notion by bringing up the way that Cyberpunk, especially from the original source, is mainly about socioeconomic interactions. They laughed.
I once sincerely believed that the drum was not an instrument, in the way that 'null' is not a number; something without which the rest is impossible to describe, but still not an instrument. I feel bad for my younger, stupider self for being so conceitedly wrong.
We have opinions on what is and isn't a genre, but what I want to know, if it is a genre or a sub-genre or a sub-sub-sub-genre, what defines it?