That's because Roy can smooth talk even the most seasoned ladies of the game. Actually, Fire Emblem has a lot younger man older lady parings and vice versa.
Oh, try to do Roy's ending with a Game Shark and "A" Rank in all the ladies he can fall in love with....
Nino was in the Black Fang under Sonia control and seeing how Sonia's Black Fang did done a lot of bad things and Nino was a Black Fang. Figure seeing that she was the "daughter" of the Blank Fang's two leaders, well, money money money! MON-EY!