I think if Ayu sang something sexual, it would be hinted, but not all out and more about the circumstances than the act itself...as exampkle I would point out a less intense form of Nana Mizukis Etsuraku Camellia
http://sziur18.wordpress.com/2009/06...s-translation/ (lyrics)
http://www.youtube.com/watch?v=RebisfWgows (song)
It hints sex, but has a main theme of forbidden love which is fairly similar to GREEN maybe.
Just like Ayu she was someone you'd never expect to sing about sex, but it turned out amazingly well.