It seems to be because a man was attacked last August while taking part in a naked bike ride event. Reading opinions in the media, some people are opposed to it in the context of indecency and child protection which is strange to me.
I lived in other parts of Europe as a child where it was normal for women to sunbathe topless without anyone being concerned. Parents taught kids that their bodies are normal and nothing to be ashamed of. What message does it send kids when their parents are outraged over something like this? A child abuse campaigner has started a petition to ban it and I see 'pervert' being used a lot. The event is planned in advance so if you don't want to see it just don't take your kids to that specific place at that specific time on that specific day.
An organisation doing a naked bike ride where I'm from would probably just be seen as some daft eccentrics which is more or less how I see it. I wouldn't participate in one and I have no interest either way but if they aren't breaking the law and not bothering anyone why would I get angry?