Why do you have to be gentle with girls?

In my gym class there is only one girl. Well my teacher is always saying be careful not to hurt her. And to not be rough with her. And in my drama class after we do games the teachers says he is glad that we didn't hurt or were rough with girls. But yet you always see girls playing rugby and rough games but yet us guys can't be rough with them! Why is that?
Why do you have to be gentle with girls?
Post Opinion