You aren't telling me they don't get a thrill out of it?
Why would they? Would you? I wouldn't. Most people wouldn't. Fucking weird of you to assume that being naked on a bike is automatically sexually exciting.
Why do they have to be naked though?
Because 'Cyclists go on group bike ride' doesn't make the news and doesn't do a damn thing to raise awareness of the issues they're campaigning about, because there's nothing exceptional or unusual about it.
'Cyclists go on naked group bike ride' makes the news every year and attracts attention because it's funny, which then provides an opportunity for the organisers to talk about the message behind it. It's a really effective publicity stunt and usually gets a spokesperson interviewed on local news talking about the issues they're campaigning for, plus people who see it go back to the office or on social media and say 'Haha, OMG, just seen a load of naked people cycling, what's that about' and then they find out about the purpose behind it. It gets people talking and it gets media attention for a cause, which is the aim.