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You get to choose your position preference, so if you want to play as a total top, you absolutely can. Each hookup has different CGs to account for playing as a top, bottom, versatile, or side.

The game also recognizes your preference and changes the inner monologue to match, so your version of the PC will comment on guys differently than a bottom player's PC does. This starts being more prominent after the first two days have passed.

I don't lock you out of any other positions if you don't always want to top, though. 

As Bucetilde666 said, there's only one main guy in the game you can't top, Isaiah. He is a strict top. Everyone else is fair game, though.