When the humans evolved from quadrupedals into bipedals (animals used to walk on 2 feet), they used to face the front side of their partner while having intimacy. (1/3)
Moreover, the Males also have this attractiveness because they get this from their mother when she feeds them with breasts. This concept can also be viewed in terms of "Oedipus complex" (3/3)
When the humans evolved from quadrupedals into bipedals (animals used to walk on 2 feet), they used to face the front side of their partner while having intimacy. (1/3)
When quadrupedal, the ass was the attractive part( they used to do the sex in doggy style then), but when bipedal, Boobs became the sexiest part for males as sex was done in missionary style most of the time(2/3)