You are very lucky you're not hurt. Falling into that curb could very easily be a traumatic brain injury. It wouldn't take much to fix that either.

I don't care what the original rationale was for that sort of grate, but I'd be on to the local council (or whatever is your city's equivalent) to have that grate changed - and I'd be filing a request to have them fix the bike, too. They should be extremely happy to be made aware of the problem before (as someone alluded to earlier) a major hospital visit is required by a member of the public crashing their bicycle.