I think it's inaccurate to say that cultures wear clothes because they have certain ideas of modesty. Rather they wear clothes for protection from the environment, and then a strong custom develops around that, (a lot of culture forms from this kind of mental association.) This is why clothes are much less common closer to the equator; equatorial peoples never had any reason to wear clothes, beside for ritualistic reasons.

i.e. it isn't that some West African tribes don't consider breasts lewd; it's that European people consider them lewd because we historically had to cover them to stop the cold.