rumrunn, our ads keep the site free to use. I understand that people do not want to view ads for various reasons and that is why we have paid memberships. Regarding how/why those ads are posted (and how internet advertising works in general):
We have "direct fill", which is ad space that we sell directly to companies, generally on-topic for the site. Schwinn or Specialized, could be two examples..
Then we have 3rd party fill, which uses ad networks to auto-generate ads based on your demographic/browsing information. Sometimes, it makes sense, like if you went to the Indoor Cycling Forum, maybe you'd see an ad for Peloton or even LA Fitness, because they fit "indoor workout".
But you can also get ads based on your browsing off-site or the browsing of people similar to you. Maybe you read a story about "[politician] does dumb thing". That's now in your browsing data and some ads are more likely to appear simply because you have [politician]'s name in your browsing history. Or maybe you looked up things like "how to bake bread", "muffler prices", and "dress shoes". Totally innocuous by themselves. But if someone else looked up those same things and also goes on a lot of "[politician] is awesome" websites or Facebook groups, ad algorithms can lump you in with them.
We do not control any 3rd-party ads and have no input on how/why they appear. I think the only things that don't appear on BikeForums are pornography, alcohol & tobacco, and firearms from Google Ads (which is only one of our ad partners). However, if you don't want to buy a premium membership and don't want to see a particular ad anymore, you can usually find a neon blue button in one of the corners of the ad or at the bottom, click that, and select "Do Not Show Me This Ad".