The female was not underage.

In 1989 the Whitney Museum and its guest curator Marvin Heiferman asked Koons to make an artwork about the media on a billboard[8] for the show "Image World: Art and Media Culture". Koons employed his then wife Ilona Staller ("Cicciolina") as a model in the shoot that formed the basis of the resulting work for the Whitney, Made in Heaven (1990–91).[34] Including works with such titles as Dirty Ejaculation and Ilonaʼs Asshole, the series of enormous grainy[35] photographs printed on canvas, glassworks, and sculptures portrayed Koons and Staller in highly explicit sexual positions and created considerable controversy. The paintings of the series reference art from the Baroque and Rococo periods—among others, Gian Lorenzo Bernini, Jean-Honoré Fragonard and François Boucher—and also draw upon the breakthroughs of early modern painters as Gustave Courbet and Édouard Manet.[36]

The series was first shown at the 1990 Venice Biennale.[37] Koons reportedly destroyed much of the work when Staller took their son Ludwig with her to Italy.[38] In celebration of Made in Heaven's 20th anniversary, Luxembourg & Dayan chose to present a redux edition of the series.[36][39][40] The Whitney Museum also exhibited several of the photographs on canvas in their 2014 retrospective.

So, it's just a dude having sex with his (then) wife. I think it's shitty art though.