Must be something pretty modern with fuel injection!

I was very heavily generalizing; there are so many different kinds of motorcycles and they vary so widely in fuel efficiency that it's really hard to average. Here in the US, the average new motorcycle sold is a 700-pound monster with an engine larger than 100 cubic inches of displacement. (Again, generalizing a bit, but Harley-Davidsons still make up over 4 out of every 10 new motorcycles sold here.) Harley-Davidson's largest model, the Electra Glide Ultra Classic, gets less than 40 MPG and weighs well over 800 pounds.

My understanding is that the motorcycle/rider combination in most cases has a very poor coefficient of drag and that's the largest issue at highway speeds.

Depends strongly on the motorcycle, however, as there are so many different kinds with varying amounts of bodywork. Some are absurdly efficient, like the Honda Grom, which routinely achieves over 100MPG.