I don't think it's super bad form. Retail/brick-and-mortar establishments have a lot of challenges these days, and people who come to try out stuff then buy elsewhere has been on the list for a long time.
Also, not sure if this has already been said but there are ways to acknowledge that you are taking up their time without actually buying an entire motorcycle. If you've already got a moto, consider buying some accessories or gear while you're there. Shit even just a hat or t-shirt (even stuff you don't really need) can show some appreciation and put a few bucks in the till. And that's not even to mention a nice tip of the 6-unit hoppy variety.
In any case, good on you for being honest.
And I agree with those who have said that a test ride is essential for a big purchase. Sure, internet and magazine reviews can give you the objective information you need but come on, so much of motorcycling is subjective and comes down to feel. I've lusted after certain bikes that I quickly realized were not for me after getting a chance to swing a leg over one.