I was very curious about this too. I never remembered seeing any ratings for the game, so I didn't know if it was rated Mature (17+ years or older) or Adult (18+ years or older) in America. Very curious indeed.
http://www.esrb.org/ratings/faq.jsp#1
I found this FAQ from the ESRB to be quite interesting actually. I thought it had been illegal to distribute games without an ESRB rating, but it only really applies to distributors who worry about the kinds of game they sell. I'm not much of a legal aficionado.
I could see a lot of people--mainly angry mothers, or groups supposedly representing angry mothers--taking offense to the game, probably more so because of the penises (despite the fact that they look more like nubs than actual penises) then the actual violent content. It's strange in America (and I'm sure in other places of the world), as movies and TV shows with violence are given moderately-high ratings, but show a pleasant scene, with two people sharing a tender moment (involving sex) and suddenly you are slapped with the most restrictive rating possible.
It's strange...why is violence tolerated more than sex? Just what do we have against penises and vulvae anyway?