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Well you've made the assumption that it "reveals something less admirable". Even saying it's sometimes is admitting that it's not always in bad interest. Instead of telling everyone that pronouning their bikes is wrong, perhaps put more focus on the people that are doing it with ill intent. I thought the whole reason you called a bike she is because it personifies it and in my experience makes me look after my bike more, as I notice it looks after me. I'm all for everyone being who they are but this seems like a very strawman argument about something that isn't harming anyone. I thought the main purpose of your way of thinking is risk prevention
You do know there's a significant difference between people choosing pronouns for themselves and people applying them to inanimate objects? Maybe you could work slowly from that starting point to the idea that sometimes the person applying that gender (usually not theirs) to the inanimate objects is revealing something less than admirable about their attitude to people of that gender. Although, if you're having trouble distinguishing between inanimate objects and people, that journey may involve a lot of tiny steps.