It is interesting that Griffith is thinking about his friend(?) while doing it with the princess xD

well
it's what is't all about

His friend is WHY he went for the princess. He'd originally planned to be much more...discreet. Patient.

Guts leaving totally wrecked his cool; he jumped the gun. It's angry sex.

Anito said:
It is interesting that Griffith is thinking about his friend(?) while doing it with the princess xD

And later he stares Guts in the eye the entire time he's raping Casca. Griffith definitely has a little more than platonic feelings for Guts, and he's one jealous bastard.