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I'm not a party to the contract between the advertiser and the content provider.
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an agreement between two other partiesIt's irrelevant what the behind-the-scenes relations are. The party you are engaging is providing the webpage with all it's content; that some of it is outsourced doesn't matter. Using your rationale, you should also be blocking any work done by freelance journalists.
If a company outsources it's billing, does this now mean that you can ethically access their services for free? After all, if a different company is the one posting you the bill, "I didn't agree to anything with them", so you don't need to pay up. It doesn't work that way. If you've ever been chased by a debt collection agency, you'll know that argument is no defence.
Perhaps walk into a diner and have a big meal. Then refuse to pay because you don't have an existing contract with any of the point-of-sale EFT providers or the manufacturer of the cash register. Those are contracts that the diner has with other parties and you have not consented to a contract with them, so you are within your rights to ignore them.
> I somehow also chose to see a specific dude taking a specific dump.
If you know a street is extremely likely to have a dude take a dump, then yes, choosing to walk down that street is accepting that you're also going to see a dude taking a dump.
Regarding your edit, I really don't care. I just think that talking about blocking ads as some sort of morally pure action is nonsense. It's not. You're consuming the content without the full bundle that the content provider put together - if you want to see the article, you're expected to see the ads along with it. I'm not saying that you shouldn't block ads, I'm just saying that if you do, don't pretend like it's actually an ethical action. If you want to be ethical, then don't use the site at all.