Sex and violence have long been a hot issue not just in video games, but also television, film, the internet and mobile content. Back in November of 1968, the MPAA formed the earliest version of the movie rating system we have today, replacing the Hayes Code which had been in use since the 30s. It has undergone several changes mostly for the sake of clarity as recently as the late 90s. It is understood that ratings systems need to change with the times to reflect the bulk of content rated under them.
Video games followed suit in 1994, responding to congressional pressures with the establishment of the ESRB and its rating system. This replaced several divergent systems that were not consistent with a single standard. Even this system has changed slightly over the years with E10+ and E ratings further clarifying the previous K-A rating.