I think it varies depending on size etc. I think it's fair to say that on average - the smaller the less sensitive.

When at fight weight as a bantamweight I don't have anything there and don't feel a thing. With more I guess it moves more so hurts more but i have yet to get someone to bash me in the tits deliberately when i haven't been dieting to really compare.

It's no where near as bad as getting kicked in the nuts is for a guy - not that i'd know from experience but i'm thinking that looks more painful.

Don't really need a rule since it's generally not a deliberate target and few boxers have aim that bad!