Toy Master Typhus said:
Why should there be a difference between punching a girl and punching a boy?
I think it's purely statistical. Since men are on average larger than women, there will be less situations which justify a guy punching a girl than the other way around. In a confrontation between a girl and a guy, on a statistical level, he will be more able to contain the situation without resorting to hitting, since he has a physical advantage.
That is not to say that the opposite doesn't happen. But, to throw a random number into the air, for every 5 situations where it's justifiable for a woman to use violence as a means of defense, there's only 1 where it's justifiable for a guy.
To the OP: We might have received wildly different martial arts training, but I can think of less violent ways of resolving the situation than "leaving him on the ground gasping for air".
EDIT: Just remembered a situation I was in a long time ago. I was once jumped at by a girl and, through a completely instinctive reaction, knocked two of her teeth out. Literally all I did was brace myself for defense and she hit my extended shoulder. I don't think anyone would even fault me for that, but back then, I was almost lynched - by a proper mob, they were even armed with tools and farming equipment. The "never hit a girl" rule is heavily grounded in society and I'd wager that's why you feel like crap, OP.