Reading always begins with an expectation or a set of expectations, I'll give you that. It's just basic hermeneutics. Whether these expectations are genre-related or not (in fantasy they're likely to be, I imagine), they're always there. But in singling out specific features of 'fantasy literature,' for example, in grouping these texts together according to what they have in common, you've already foregrounded what is least interesting about them, you've already created a sort of homogenous zone of 'fantasy' that you can travel over in any direction because you know it already, without even having to read it, because it's 'fantasy.' Just as a sort of example, look at the idea of 'Romanticism.' 100 years ago (well, technically about 105) there was no such thing--literary historians hadn't invented it yet, hadn't yet assimilated the very disparate texts we now think of as Romantic into a whole. 50 years ago, any schoolboy could have told you what Romanticism was. Now we've come full circle. In fact pretty much the biggest "scandal" (used loosely) of English departments in the last ~40 years was the argument over whether there was or was not any such thing as Romanticism, and if it did exist, just what exactly was it?
Anyway, my point here is only that genre comes after the works it describes, so to give it any sort of authority over them is backwards. It is absolutely possible to read without any idea of genre in your head.