Alternate view, I hate sites which require login & password, especially to only view content. I try to ignore every such site, because those are designed really badly. As well as I don't get it what's the correllation between email and registration. I don't want to give my email, nor I want to register. If I need to give email, I can give any random temporary email. Signup procedures on many sites are horrible. Best sites do allow at least accessing content without these hindrances. Logging in, especially on mobile, is painful anyway. Using some kind of federated login is privacy issue (in most cases), so it doesn't solve anything either. For lulz, how about just PGP signing nonce? They can verify it against my public key. PGP also allows me to easily create as many parallel identifies with strong authentication as I want to.