I often wonder about women who no matter what occasion or season have the intermammary sulcus on display. The majority of females leave something to the imagination. Do they think they are on their own "reality" program?
Well, sure, in some cultures nudity in general is no big deal. That doesn't mean no body part of the opposite gender attracts attention there. You'll find examples all over the world of breast sexualization, so it had developed in many places independently. Signaling of sex characteristics around the breast area is also found in Gelada Baboon females, which mimic the red patch around their genitalia on their chest as well. In all likelyhood, attraction to this region goes back millions of years.