As far as I'm aware, the Mapplethorpe photographs I've seen show no explicit sexual acts whatsoever, and flipping through my copy of his largest book (Mapplethorpe) bears this out -- although some of them do indeed show the erect male anatomy. As 2F/2F stated earlier, the vast majority of his work consists of portraits and still lifes. If one defines pornography as intending to elicit sexual arousal, then pretty much none of his work would qualify.
I'd suggest that anyone curious about his work visit their local library for:
The Robert Mapplethorpe Foundation, Mapplethorpe, Random House, ©1992.
Richard Marshall, Robert Mapplethorpe, Whitney Museum of American Art/Bulfinch Press, ©1988.
Or, if you prefer to not see any of his controversial photographs at all, books and calendars are plentiful that feature only his more mundane subjects.