Quote removed
You're right. Its cars only. White Dalton (Bikers lawyers) have this
The short answer is that if you have your bike serviced by a perfectly reputable, fully qualified mechanic using all the appropriate parts and specified fluids, the exemption that applies to cars .... does not apply to motorcycles.
However, there could well be arguments that such a guarantee contract is unfair and if you felt strongly enough about it you could refer the contract to the Court, but if I were you, I would get my servicing done by the franchised dealership as you would have a massive fight on your hands with a well resourced manufacturer. Most dealerships survive on the servicing and clothing sales. The margin on new bikes is genuinely tiny.