Todd Gilchrist editor From the wah-wah guitar that opens the title track to the operatic closer “Just to Keep You Satisfied,” Marvin Gaye’s 1973 album “Let’s Get It On” expressed the joy — and complexity — of human sexuality like virtually none in popular music before it. To commemorate the 50th anniversary of its release, Motown/ UMe is holding an event Wednesday featuring fellow R&B luminaries Smokey Robinson and Jimmy Jam, Gaye biographer David Ritz and moderated by UMe A&R vice president Harry Weinger, in advance of a new deluxe edition of the album.