Males only please...Is this a cultural influenced reaction or instinct?

Fluther women: Please refrain from responding. I am only interested in the male perspective.

Fluther men: There have been many questions posted that pertain to women going bra-less. It often sparks responses by men that they look and cannot help it. The same applies when it is a woman in a low-cut top.

Is this reaction an instinct or is it conditioned? If you have ever lived in a nudist colony, in a place where women do not wear bras or go topless, or if you are gay, I would love to hear your opinion, but am open to any man’s perspective.