However, I have always thought that breasts are not seen as 'sexy' (I have not expressed that at all well, just hope you know what I mean) within many African societies.
Interesting question.
Possible reasons:
Africa is big. There can be cultures that view breasts as sexual, and those that do not.
Or the breast-ironing is a relatively new phenomenon, that is a reaction to the sexualization of breasts by media and the ensuing male behaviour.
Or breasts are not seen as sexual, but are seen as a sign of adulthood, and the traditional rules only protect female children without breasts from male sexual violence. (No culture considers menstruation as sexy, but the onset of it is often used as excuse for forcing children to marry and submit to rape by adult men.)