The sexual revolution of the 1960s and 1970s in the West spread globally and penetrated Africa. Some of the traditional and very sound cultural restraints that were present to curb promiscuity were broken down. Had this been in the 1920s or 1930s, I think it would have died out much sooner. Even in polygamous marriages, it was a contained unit. Urbanization has played a very significant role, because families have had to be separated.
Men leave their families for up to a year.
While they're away, the temptation is very, very great. Another is the displacement of people because of war. With that comes the complex nature of troop movements and their actions in camps. Some armies are notorious for promiscuity and rape. The health issue is another one.