Looking for scientific answers..

It's a matter of association with the exposure of them. Another, less known, part of the body that tells us something about a person we're speaking with is the inside of our wrists. When people are more open to the person they're speaking to, they are more likely to show them when moving their arms, and when they have more reservations toward the other, they are more likely to hide them. By nature we feel more exposed when showing that part, and others subliminally 'get' that we're more free and open with them when we don't mind exposing them a lot. It sets the atmosphere of the communcation. There are more body parts that tell us something about the nature of the communication depending on how easily they're exposed. We automatically take these things into account and act accordingly. For a woman to freely expose her breasts to a man usually takes certain conditions. Instinctively we take possibilities into account and allow the brains to imagine things that might be possible.
Nakedness of various body parts is an important part of body language and sends subliminal messages we pick up on instinctively. We can be more sensitive to some signals than others, but we're all at least slightly influenced by these things. Some communication experts also teach about how to consciously send messages this way to manipulate communication, but we all do this at some level anyway.