Our Gemara on 54b and 55a analyze a Biblical idiom את מקורה הערה “her source was revealed “ (Vayikra 20:18) to teach that the violation of prohibited sexual intercourse is defined by the initial entry, which is defined as some opinions as merely contact of both sexual organs, or the entry of the glans into the vaginal canal.
The actual derivation and etymology of the phrase requires some analysis. Rashi (Bereishis 42:9 and Vayikra 20:18 says the word He-Ara means to reveal or uncover. The presumable Hebrew root is the same as to awake “Er”, as the consciousness is revealed or perhaps the eyes are open, and “Ta’ar” a razor, which uncovers and reveals skin (see Gesenius’ Biblical Lexicon “ערה”). For those of you who find the pictographs that seem to be inherent in Hebrew letters and roots interesting, you may note that Ayin is a pictograph for an eye, and Reish is a pictograph for a person. Reish=Head or person, thus, the root implies a person wide open or wide-awake.
The idea in peshuto shel mikra terms is that the person has done an unseemly thing, revealing an area that should be private and covered. “Mekorah”, “her source” according to Hakesav Ve-Hakabbalah (Vayikra 20:19) refers to the vaginal area.
Ramban explains the usage of this phrase in regard to being sexual with a Niddah. He states (Vayikra 18:19):
It is possible that this is the sense of the expression, he hath bared her fountain, and she hath uncovered the fountain of her blood, for that blemished fountain should be covered, and not bared in order to draw its bad and extremely harmful waters. Thus intercourse has been prohibited to the holy seed all the days of her impurity, until she immerses herself in water [i.e., in a ritual pool for then she will be purified also in her thoughts and become completely clean [in body and mind].
As we often see in Psychology of the Daf, psychological ideas can be expressed through mystical terminology and vice versa. Here too, Ramban is expressing the sacredness of sexuality in terms of a private interior experience. A Niddah, like other forms of tuma is related to loss of life force and related trauma. After all, the menses are the loss of a potential child. The feminst psychoanalyst, Karen Horney, published a paper on "premenstrual mood swings",on irritability and anxiety, the listlessness, self-depreciation, or even outright depression,which many women experienced in the days preceding menstruation, linking them to strongly rejected fantasies of motherhood. (Karen Homey, 'Premenstrual tension', in idem, Feminine psychology, London, Routledge and Kegan Paul, 1967, pp. 99-106.) There us a need to shed these feelings of trauma and impurity through the buffers of rituals to guide us through liminal events. To be sexual with a Niddah would be a terrible mixing of psychic forces of creation and death.
This phrase is used by other forbidden sexual relations as well. The very non-modern idea is that when people are cavalier with their sexuality and reveal too much they will feel empty and depleted. Sexuality is a powerful life force that many ancient religions intuitively considered sacred and worthy of conserving. It is only the “advanced” modern folk who have the hubris to think that such powerful human drives and force be disregarded and used with little care for how the person ultimately feels. Modern sensibilities have utterly failed to protect people from unchecked sexual impulses. The number of cases of sexual abuse by persons in power seems to have no correlation to how modern thinking the perpetrators are. Governors and so-called great leaders fall prey to their appetites and power, and the victims suffer. The Torah knows how important it is for sexuality to be a private and modest experience.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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