Sitting on another rider's motorcycle without their permission is generally considered inappropriate. Motorcycles are personal possessions, and riders often have a strong attachment to their bikes. It's important to respect other people's property and boundaries.
If you're interested in sitting on someone else's motorcycle or have a genuine reason to do so, it's best to ask for their permission first. Communicate your intentions and explain why you would like to sit on the bike. Most riders will likely be understanding and accommodating, but it's always courteous to seek their approval before touching or interacting with their motorcycle.