I'm not at all into Harley bashing but ever hear of the 'ol saying, "..run what you brung..."?
If the dude solicits the action, and the other person accepts the challenge then both deserves the outcome either way and it shouldn't matter what they ride at that point. To dismiss the challenge just b/c the losers bike is in a different class is kinda calling "sour grapes"...In this case, it certainly sounds like the Harley guy put a challenge out there, shame on him for not sizing up the competition correctly...no foul on the play, he put it out there...
Now accepting the challenge in the first place from another bike which you feel is out of your league (above or below) is a whole n'other story....
Whenever I was out on the Busa, I had countless challenges. I opted to not engage in most. The ones I did accept were either other Busa's, the ZX14, or litre bikes that were cocky...any talk about this or that being the reason I or the other person loss was taken as pure whining...