It is illegal to drive beside a motorcycle?

No, as long as you are obeying the rules of the road, and not crowding them.

My opinion: it may not be illegal to ride beside a motorcycle but as with a car, you are cutting off a means of exiting you lane should an emergency arise ahead of you and you have to shift lanes quickly. Where would you go?

It is also very annoying to me to have someone just sitting there matching my speed.