I dont think anyone considers boobs as purely sexual. Due to their biological function they are also distinctly maternal, but within the context of sex i suppose they are considered erotic because full breasts biologically indicate good health, and the ability to successfully nurse children.
Same reason we find nice hips attractive although it serves no immediate purpose within the context of sex. But they are also called child bearing hips for that reason; we find them attractive because they indicate a mate able to successfully give birth.
I mean, the tits thing is a myth. I’ve seen women who were basically flat nurse children, and women with back problems on their chests have trouble. the actual amount of tissue required is very small, or something.
