How erotics is born? Why woman's bottom looks attractive and erotic to man?

How do we recognize that something is sexy?
Why do we get aroused by thinking of naked women?
Why we find some things and somebodies sexy, and others - not?
How do we learn to recognize sexy things?

How men learn to get excited about seeing naked women?
How do we learn that this images should make us horny?

How do we, heterosexual, get attracted to girls and not to boys for intimacy and sexual activities?

Questions, questions...

It's easy to answer: "it's genetically encoded", "men recognize women with high probability for fertilization", blah-blah-blah...

Some researches suggest that brain is prewired to see the images of specific bottoms, breastes, thin waist etc. as "hot", without being teached.

I agree that many aspects of beaty in general probably are "prewired" in brain's design. However, I can't get no satisfaction from such explanation - I need something more specific, so I've been searching for something that could be captured from the environment and used to configure the brain, without having specific "Ass-recognizing hardware" built-in.

Some time ago, I had a hypothesis about this software way for configuration:

Ass curves visually resemble man's penis, seen from above. Thus the way boy and young man's brain is configured to see this images as erotic could be the following - whenever he sees his errected penis and this image, he's horny... Thus, this image is learned to be an indication of horniness... :)

It's also true that while seeing a naked woman, a speculation about sex could be made, even if the boy still doesn't have explicit knowledge about what really sex is. Even young boys know, that there are two types of people, which are different; they marry and there is some kind of secret about how they were born. Even little boys also know, that certain parts of women body should be hidden, and when thay see nudity there, they could suppose this is an indication of something specail, unusual. If the boy has a vague idea of sex, but knows that it is something special between man and woman, he could guess that nudity is connected.