Often we complain about mankind being sex-obssessed and lustful, but need I remind you: we are born with sexuality, and the drive to pursue it. Same with all creatures. It goes back to the very first life form that appeared on Earth.
The first life forms on Earth were probably bacteria in the sea, but even they were driven by the need and desire to reproduce and procreate, and all creatures that would be born would inherit that same sexual drive, and that drive would evolve in different ways, as would the manner that creatures express and fulfil them.
You can see the influence of billions of years of procreation on human culture, including religious and spiritual culture. Sexuality has been a part of human life and culture since the species began. For instance, there are phallic stones of a spiritual importance in the pagan cultures of Europe, as well as in Japanese religious culture, such as Shinto, and many other countries. An example of that is the stone on the Hill of Tara in Ireland, where potential kings had to drink ale and symbolically mate with and marry the goddess Maeve to prove he is worthy of kingship.
Even today, Hindus venerate the Linga, a sacred stone that represents a very special part of the god Shiva.
Breasts were also a symbol of fertitlity in ancient culture. Of course, most ancient cultures had different ideas of ideal female beauty, so they didn’t idealize the same women we do now.
Of course, when Christianity began it’s puritanical campaign of sexual repression, those images were largely banished, but not completely gone. For you see, we are a sexual creature, and so will always crave sexual images. So they hide sexuality in the form of less overt expressions.
Nowadays, we are loosening up, but there’s still some “moral concern” among people who just don’t realize how humans work. We will always crave the sexual, as a species, and no thousand year campaign of sexual repression will ever change that (in fact it’s probably made things worse). But relax. I’m not saying we are all ultra-perverted monsters. All I’m saying is that we have a natural fixation, and something of an obsession, towards sex and sexuality, and towards sexual imagery and metaphors.