I do not think it was “decided” that “sexy parts” are “sacred”.
And also being sacred does not mean “bad to touch, look and even talk about them.” Sacred just means set apart for God.
And also not all religions see sex as taboo. For example, in Hinduism sex is celebrated and from whence you have the karma sutra.
But in the biblical narrative of the creation, man and woman were naked at the beginning, which I suppose, is like the way animals are naked today, and have no sense of shame about it, even to copulate in “public”.
It was only after Adam and Eve have eaten of the fruit of the “tree of knowledge of good and evil”, ie only after they have attained knowledge of good and evil, were their “eyes opened” and then realised that they were naked, and felt shame, and covered themselves up.
Now if this narrative is true, then we, unlike animals, have a natural sense of shame when we see others’ nakedness or when our nakedness is exposed before others. And this sense of shame is somehow derived from our knowledge of good and evil.
Perhaps we are ashamed because, now knowing evil, we immediately abused the sexual pleasures suggested by our “sexy parts”, such as desiring sex for sex sake, rather than sex being enjoyed as part of and the expression of the ecstacy arising from the love and oneness with another.
But I am not entirely sure why we feel ashamed about nakedness and what is the psychology behind this, and I am sure someone have explained it somewhere on the web, eg the psychology of shame, which I will search for right now.