I guess the problem is, that anything which can effectively stop a motorbike, will also be problematic for a lot of other users, including us cyclists.
I can't really see how you can effectively stop motorbikes, which after all did evolve from bicycles with the addition of an engine, so are by definition subject to similar limitations in terms of manoeuvring.
The only real difference I can see between a pedal-bicycle and a motorcycle is that the motorcycle tends to be heavier, although you can't really build a barrier based on that since it'll almost certainly also inhibit people in wheelchairs etc.
And of course, even if it's possible to get a pedal powered bicycle through and not a motorcycle, invariably the barrier will block tricycles, recumbents, tandems, trailers, bicycle with panniers, wheelchairs, puchchairs and prams, and probably other things which I haven't thought of.
I'd love to see a design which just blocked motorcycles, but I'm damned if I can see how you can do it, short of a security camera and ANPR (defeated by blocking or removing the number plate of the motorbike). Image recognition and an electronic gate ?