Why does anyone care? Let people mark things as "adult" and move on. Youtube does this mostly, Blogger does this, why can't Yahoo and Tumblr?
So if you are paranoid and don't want to log in and start getting those videos in 'recommended for you' list. Just take the embed code put to it a .html file and open.
data:text/html;charset=UTF-8,
to the beginning of the code and paste it to your address bar.Expecting that minors will respect an "adult" label is laughable, and anybody suggesting so knows it. It only takes a moments ill conceived curiosity to see something that will stay with a kid (or a person in general) forever.
Personally I hope that a company currently lead by a woman takes a hard stance on this issue.
Also note the definition of pornography does not include nudity.
(For the record, I'm pro-porn and pro-sex, sex is great!)