Though its origins can be traced further back, it was WWII that really put pinups on the map. The pinup was a reminder to troops of what awaited back home.Look at it how you want but the fact is it kept soldier’s morale up. on the plus side it also served to launch the careers of many a young Hollywood starlet.
"It’s an art form expressed through performance, photography, fashion, music, tattoos, etc., that is with us to this day. It’s taken a decidedly more alternative bent in recent years with the popularity of Bettie Page, Dita Von Teese, Suicide Girls, etc."
I personally think it has a little something to do with pirates and how they had mermaids/sirens on their boats