The two movies I noticed that have erect penises that are R are the South Park movie and Scary Movie, but the penises are detached.
This is the thing with a penis (holds out hand with fingers curled, and slowly flattens his palm), its like R, R, R, R, NC-17.
You just cant show erect penis.
Anyone who follows movies with any kind of regularity knows that the MPAA has some oddly specific qualifications when it comes to rating theatrically released films.