From algebra to medical science, astronomy to astrology, law and finance to creative pursuits of artwork - everything that so called  ''modern science'' has ever achieved was partially if not wholly, procured from textbook of ancient civilization. While some may agree with these facts of derivation, some may simply not. But whatever you believe to be true at microscopic level, facts still point-out the amalgamation of ancestral things into our day to day lives.
That textbook of ancient teachings was inscribed with a single point of view i.e. to educate the upcoming generation. But novel discernment subdivided it into two parts: ethical and bizarre.
The ethical part consisted of language, their system of governance, means of trade, farming, etc which contributed in growth of either society or an individual. And the unethical or taboo part - well among several, the one you're about to read tops it all.
Ancient civilization
'SEX' a common yet controversial topic in itself occupies a basic and perhaps a significant role in human life.  While pleasure and duty both are associated with this term, the ancient men had a different perception on matter of intimacy then ours. Ascribing sexual practices with God might be one of those reasons that resulted into this vast indifference.
This wide gap between our sexual practices and our forefathers custom, could perhaps be downsized by evaluating the differences which could buffer between our sexual morality and prejudices.
Adams apple
Ethical or taboo, sexual practices of ancient men could raise eyebrow for today's laureate society. Among several, we present you with top 8 customs that may sounds bizarre but have undoubtedly laid the foundation for inordinate success of modern porn industry.
8] Wife as a commodity:  
Wife as commodity
The term ‘commodity’ associated with women, especially when one plays a vital role in sustaining the society, may reflect upon as unethical and showcase of cheap mentality per present norm. But it certainly wasn’t for pre-Islamic Arabs.
For them ‘Wife Lending’ was traditional and pretty straightforward way to enhance their legacy. An efficient means adopted by families who wanted to elevate their rank and stature in society.
The protocol was simple: Married woman would leave her husband’s house and stay with a man - until she got pregnant. The chosen one was no ordinary man, but possessed the attributes of greatness that family sought after.
Children born through this procedure could only be claimed by the husband if he abstained from any kind of sexual activities in the absence of his wife.
Group sex
Similar form of practice could be reported from populated regions of Himalayas. Cultivable lands being fewer, and for families with more than one son; dispute over division was a big issue to solve. The solution: Finding a single wife for all the sons in a family so that the land could be kept intact. Time allocation for spending intimate moments with a single wife and individuals were laid before as a mutual agreement.
Anthropologists call this practice as “fraternal polyandry
7] Sex slave and prostitution: 
Sex slave
Unlike today, prostitution and sex slavery were not means to make money in those days. The Romanian law regarded slaves as a distinct property. Forced sex with slaved person wasn’t considered illegal and certainly not as “rape”. Threesome and foursome were also secretly followed as means of debauchery.
The Assyrians believed that for a woman to receive her grace; she must have sex with a stranger. Hence, every woman in Assyrian empire had to present herself as a prostitute. After she had an intercourse with a customer who bid her first; the women was considered as holy, and was no longer forced to have sex.
Spiritual form of prostitution can also be witnessed in the Deer Horn Muria tribe who live deep in Central India’s Chhattisgarh region. They practice something called Ghotul, which is a festive mingling of teenage men and women to teach them songs, tribal dance... and more importantly an open sex.
At night, they engage in ceremonial orgies and sexual romps. Girls drink a natural liquor as a herbal contraceptive to avoid pregnancy and then choose different sexual partners every night (no shame in getting sex frenzy). If the herbal drink doesn’t work and the girl conceives, the entire village would contribute in nourishment of the newborn, since no one knew who the actual father was.
6] Fuck, but do not ejaculate:
Taoist sex
Taoism lays it spiritual belief on a mysterious force “Chi” that is supposed to inhibit matter in any form - living or non-living. Qi (or chi) is divided into yin and yang, and are thought to maintain the spiritual harmony.
According to Taoist, Qi or  jing (semen) was responsible for sound body and mind. Practitioners of this conviction avoided ejaculating too much of their semen during sex.
Contrary to this, it was also believed that one of the most effective ways to get more jing was to have more sex with beautiful virgins that to without any ejaculation.
5] Dildos:
As the tales go - Cleopatra was the first ever to use a vibrator. The ancient Greeks and Roman also loved their toys. They had leather sheaths covered over wood or stones so that a better simulation of an actual penis could be achieved.
But if leather – sheathed dildo wasn’t available, the obsession for sex-toys soared so high that Greeks would simply bake loaves really hard and used them for intimate satisfaction.
4] Homosexuality was a custom:
Sex between two males was neither considered a homosexual act nor the participants were labelled as 'gay'.  Sexual desire was distinguished by the role that each participant played in the sex act, either of the active penetrator (dominant, higher status) or the passive penetrated (submissive, lower in rank).
Pederasty was socially accepted without any disgrace or shame, because the adult penetrator was believed to be bestowing the young boy with manly traits and knowledge that he already possessed.
Same sex
More bizarre form of homosexuality was followed by Cretan man. When a potential male lover was spotted, a Cretan man would kidnap and secretly lead him away from civilization. While away, they would exercise  masculine activities including fishing, hunting and sex; but all this happened as a part of legal tradition popularized at that time.
3] Reverence through public masturbation:
The ebb and flow of the Nile was thought to be  originated from their creator's (god) ejaculation. This idea caused the pharaohs to ritually masturbate into the Nile to ensure a wealth of water for crops. During the Egyptian festival of the god Min, who represented the pharaoh’s sexual power, men regularly masturbated in public
The Pharaoh would walk to the edge of the Nile, get naked in public, and masturbated, with particular emphasis on making his semen fall in the river and not on land - so as to help Atum maintain the life-giving force of the Nile and keep it flowing.
2] Oral pleasure:
Oral sex or fellatio dates back to Egyptian myth of Osiris resuscitation. When Osiris was chopped down to death by his brother, Seth; Isis walked down on a treacherous path to bring life back into her lover. In her quest, she found all his body parts except penis. To complete Osiris, she created a penis out of clay and blew life into it.
A tale thus, formulated spiritual belief  of fellatio for Egyptians and Phoenicians. Romans were also well-known to have their slaves or prostitutes appease them with oral pleasure.
1] Sex with animals:
sex with animal
Bestiality in ancient times was so common that bible sentenced death penalty for those who were found having sex with animals.
According to Herodotus, goats were said to be part of this religious practice, and evidence of which can be best understood from an idol attributed as Pan copulating with a Goat’, probably the best-known objects in the Naples Museum collection.
Roman mythology is inscribed with facts depicting the custom of sex between men and animals. A carving on a bone rod dated to be 25,000 years old shows a lioness licking the opening of either a huge penis or a vagina. Another early depiction of bestiality was a picture of a man having sex with a donkey found on a cave painting in Italy from the seventh century B.C.
The ancient Greeks and the Egyptians were also into bestiality for religious reasons. Number of researchers even agree upon tales of Egyptian hunters killing a female crocodile after having used her as a sexual medium.
This really shows as to what extent a human may go to satisfy his quench under the name of tradition or spiritual practice.
All these ancient sexual practice may put forward a smutty and obscene mental image of their respective followers. But what we do fail to understand is this: Birth of mankind is primarily for two reasons - to reproduce and to sustain. The sustenance part is well maintained and revolutionizes as the tribe grows in knowledge and facilities. In similar manner the reproduction part has to go hand in hand or else the results would be catastrophic.
At the end instead of pinpointing, it should be recognized that we are what are ancestors have left behind, even if that means our continuation with some outlandish sexual practices like oral sex, homosexuality, group sex or even prostitution.

The god in me and the beast in me

And all deep things come up to light;
And I would barter my soul to be
The prize of love for a single night....

-  George Sylvester Viereck, “Love Triumphant"