I can testify to converting from only ever topping. I knowingly never took a single load in my arse before converting as I almost never bottom. Since converting two years ago, I have taken one poz load in me.
My theory in how I converted is outlined above, It could only have been from fucking a bottom (or bottoms) who had poz cum inside them already when I fucked them. As I like group sessions, and attended a few where this was the case, then this can only be the cause of my infection.
I had a neg result at the GUM clinic two weeks before conversion, and a positive result being given from a test taken at or about the time I had the fuck flu. That narrowed it down to one or two possible group sessions within the previous month.
Might be a small chance for a top to get hiv, but I've proved it possible!! (I am uncut if it makes any statistical difference)