Are you only talking about snakes? I think male iguanas have a tendency to be more aggressive, especially during breeding season.

With my snakes I couldnt tell ya. Ive got some males and some females, but not of my snakes have ever really been agressive. Last time I was bit by a snake was at least 7 or 8 years ago by a WC texas rat snake when I was still living down there. Id say it all depends on the situation, species, and individual snake.