toplessness why is it acceptable for men but not women?

It's only in Westernised societies that public boobs are unacceptable. The first reason is extended cold seasons and practicality, and the second is prevailing cultural attitudes.

Breasts in the West are increasing not used for their primary function - feeding children. The disgust shown on breastfeeding threads here is testament to the fact that people are often uncomfortable with the breast being used as anything other than an aesthetic primary sexual characteristic, rather than primarily a functional sexual characteristic.

I'd say most of this is to do with portrayal of boobs in the media, fashion, and the increasing sexualisation of society. The price you pay for Wonderbras and Maxim is in the commodification of boobs, and the price of a scarce commodity is always higher, so the less you see, the more you want to see.

Twenty minutes on a topless beach and abundance makes the experience dilute. A quick glimpse of side-boob in a bar when you haven't seen any in a long while means it's an event.

ETA: Western religions are pretty hot on sex guilt, further adding to the 'dangerous' element of boobs without rules :P