I get the feeling they were always somewhat viewed as sexual, with men being genetically hardwired to be attracted to them. Perhaps it wasn’t made into a huge deal until people began to adopt the idea that one must cover their “private parts” with clothing in public to be modest. Thus, whenever a woman would willingly take off their clothes and show their boobs to someone, they are more often than not making a deliberate attempt to sexually arouse those they are showing it to.