I think it all depends on the context. I don't know what the motorcycle rally you went to was like. I know some people might think of rowdy biker gangs and lots of rowdy behavior. But I know that's only part of the biker community. The event could just be bike enthusiasts showing off their bikes in a very family friendly atmosphere. I'm not a biker, but I went to one show. Some parts of it weren't exactly what most would call family friendly. The T-shirts in a wet T-shirt contest soon came off. Another contest had women (on the back of a bike) riding under a hanging hot dog and trying to bite it. You can see the symbology and can imagine some of the chants from the crowd. There was no "event area" that you had to have a ticket to get to. This was all happening in an open parking lot within 20 feet and in plain sight of a downtown street in a major US city. I did see a more than one kid under the age of 13 watching the topless women.
I've known JWs to participate in automobile shows. I've seen them go to air shows too. A motorcycle rally could be very similar.
What I do know they frown upon is anything that's a "worldly event" where you'll be exposed to anything they don't like. For example, rock concerts, political events, a comedian who's known to swear a lot, etc. If the motorcycle rally had a bad reputation, I know they'd frown on it.