> I'm not aware of any large ISP that has throttles or bandwidth caps
Really? I've heard countless stories of US ISPs doing deep packet inspection to throttle torrents and other peer-to-peer traffic. If they've stopped that, my googling suggests it's primarily due to FCC regulation.
> You're forgetting that it's by default on, and that one must call their ISP and "opt-in" to seeing porn.
Ah, incorrect. All major ISPs (a) ask during sign-up, and (b) provide a web-based tool for turning it off. There's no need to ever talk to anybody.
> Social stigma aside, how can you realistically and with 100% accuracy block only true porn websites?
You can't, but there's already products on the market which claim to provide content filters which block porn. Net Nanny is a personal one, a good many companies implement one.
> And who's to say a website is illegal or not - I for one certainly don't want my government telling me some download sites are legal and others are not.
Your Government actually decides that more websites are illegal than any other country, delisting them from the .com DNS space for everybody, whether in the US or not. Perhaps you should fix that before worrying about an optional content filter that nobody in the US is currently talking about creating.
> Not to mention who's to say I intend to break the law simply by visiting a website?
Nobody. It's an optional content filter.