why did roy get mad at nathan during the sex scene?

Before in the school bus Roy had asked Nathan if he had ever fooled around with anyone else before and Nathan said no.

In the car he thought Nathan's actions had been learnt, as if he was experienced and Roy became paranoid that Nathan was a slut or something .. that's how I took that anyway. It turned out Nathan's dad had sexually abused him throughout his childhood, unless I misinterpreted things.

Somewhere in one of the other Dream Boy discussion threads this is dealt with pretty extensively.

Roy assumes both he and Nathan are equally innocent, or at least in experienced. When Nathan has butt sex with Roy in the pick-up, that implies to Roy that Nathan has experience in matters of sex, and I think Roy feels a bit intimidated.

Some said, that Roy might see Nathan as an experienced SLUT. I don't think it goes quite that far. I think Roy is just intimidated that Nathan might have more experience than him. And perhaps a sense that there are aspects that Nathan has not told him about, which makes him feel a bit betrayed.