A few days late, but I just found this news item. It seems faintly ridiculous to me. Below is one of the album covers involved, and I absolutely fail to see how that can be construed as sexually explicit (okay, so a group of young naked people running off into a wooded area may just about imply the possibility of sex, but that's sexually implicit, rather than explicit, and it may equally well imply the possibility of going swimming in a river). If it's not sexually explicit, the idea of classifying such nudity as "adult" strikes me as rather unfortunate.
The other example shown is sexually explicit, but it's very stylised, does not show genitals, and is clearly intended to make an artistic/political statement rather than to titillate.
To sum up my view on this (as the interpretation given in the article is that Google is trying to dissociate itself from pornography), nudity =/= sex, sex =/= porn and porn =/= harmful (okay, so a lot of porn is harmful, for a variety of reasons, but it's not harmful just because it's sexually explicit).