The short answer to your question is "pretty early".

Early Christians were repelled by Roman sexual practices, and the early apologetic literature contrasts Christian chastity with the license of the "pagan" cultures around them. The legitimization of Christianity in the Empire was closely attended by bans on male prostitution (Philip the Arab), sex with children and slaves/concubinage (Constantine), then all prostitution (Theodosius), then all homosexual acts and heterosexual fornication (Justinian). the Code of Justinian codified most of what we think of as Christian marital and sexual strictures into law.

Also, I think you're underestimating the impact of the sexual proscriptions in the Bible. The New Testament contains numerous condemnations of sexual immorality (Romans 1:24-28, 1 Corinthians 6, 1 Thessalonians 4, Matthew 5:28, Ephesians 5:5, Hebrews 13:4, 1 Corinthians 7:2, Colossians 3:5, Galatians 5:19, 2 Timothy 2:22, 1 Peter 2:11, among others) and some of Paul's harshest language is reserved for those who violate sexual norms.

Sources: From Shame to Sin: The Christian Transformation of Sexual Morality in Late Antiquity

Prostitution: An Illustrated Social History

Defending Constantine: The Twilight of an Empire and the Dawn of Christendom

Slavery in the Late Roman World, AD 275–425

The answer is right at the very beginning. While the Gospels do focus on sexuality quite a bit, Paul is obsessed with it in his Epistles that are our best source on the earliest Church.

When English translations of the New Testament bible talk about 'sexual immorality' they are really translating the greek word porneia (πορνεία), it’s used almost every time the topic of sex comes up and often when talking about the worst sins in general. If you can really grok what Paul was talking about as he uses the root for the word over and over again (it appears 32 times in the New Testament) then the rest falls into place. Now porneia has always been translated into Latin as fornication, while being understood by many conservatives to just be a 1:1 stand in for 'any sexual expression not between husband and wife'. However, Porneia in post-classical Corinthian Greek did not mean generic sexual sin, or even sex outside of marriage, at all and neither did fornication in actual Latin.

The word family of porneia (πορνεία) as the Classical Greeks actually used it was related to the verb to sell, and was only ever used in one context. A porneon was a house of forced prostitution, pornos (πόρνος) were those who sexually assaulted those forced into prostitution, the pornēs (πόρνης) were specifically those prostitutes who were sold for a pathetic sum to any taker, and itinerant sex traffickers were called pornoboskoi, a singularly unpleasant combination with the verb that described the herding of livestock such as cattle. The word porneia itself is a pretty weird conjugation and only has four attested uses before Christianity, each time referring to the general concept of one's body being sold as a sexual object. Paul used the root word repeatedly in his Epistles while making two primary assertions, that the ubiquitous system of porneia (πορνεία) fed by war, poverty, and callousness was fundamentally not OK, and that a laundry list of examples were pretty much the same thing as porneia. This fundamental position on heterosexual sex, that it is something that even could, much less must be divorced from exploitation was profoundly radical and novel for the time - even if it is hard to see today being the water we swim in. Paul was clearly very concerned about sex, so much so that he comes from an almost totally ascetic perspective, and indeed with modern eyes that he was such a large part of giving us sex was one of the most fucked up aspects of the world he lived in, where the scale on which it was fucked up is truly unimaginable to us modern readers of the historical records we have.

Lest you think there is significant doubt about what the concept meant to the Classical and post-Classical Greeks, the porneia word family communicates one of the more thoroughly defined ideas that we have from their lexicon, as the ancient greeks were so legally concerned, as well as facetiously fascinated, with it. To really understand it requires a little bit of context. Under the laws of Draco in ancient Greece (621BCE), where we get the term draconian today, any man who caught another man sexually violating (adultering, μοιχεία) his wife could legally kill that man with the same immunity as an athlete who accidentally killed someone in competition {23.53}. Consent, or even any action or feelings on the part of the woman in question, were perfectly immaterial to the crime that one man committed against another that this was. This meant that, in addition to being able to just get some friends together and safely jump him while he was taking a shit Pulp Fiction style as Draco allowed, the cuckold could also capture the adulterer and inflict whatever tortures he imagined so long as he didn't use a knife{59.67}. In practice however, this usually resulted in the aggrieved man privately extracting exorbitant amounts of money from the adulterer in exchange for publicly forfeiting that immunity, but it also formed the basis for some really fascinating trials. Under the laws of Solon (594BCE), as well as later codes, this legal vengeance only applied to wives (as well as concubines kept for the purpose of producing free children) and explicitly not to women available for sale{59.67}, pornēs or those like them such as flute players, two-obol women, bridge women, alley walkers, or ground beaters. Thus we have solid records of those accused of adultering wives aggressively defending themselves by declaring the objects of their attentions to be pornēs - while very precisely defining the term as describing women available for sale to any john, particularly if at a fixed price.

Its important to keep in mind what sexual immorality - porneia - meant for the society that Paul was advising his churches on how to live in. Before Paul, porneia was seen as a totally uncontroversial part of life, a public good, tradition even held that Solon the lawgiver even opened a brothel in Athens himself as an act of public service {Philemon, Frag. 3; Athenaeus, Deipn. 569d}. The systematic rape of the vulnerable that the institution represented was regulated by cities in the same way that roads were, as a lucrative and essential public utility. Indeed the task of overseeing the institution was given to the Astynomoi, who were entrusted with tasks associated with maintaining thoroughfares such as ensuring the reputable disposal of shit and abandoned corpses from the streets. A price caps of two drachmas was established to protect 'consumers' and the same officials who enforced it would also adjudicate disputes over women (by the drawing of lots of course, the women themselves were not to be consulted), pimps were given licenses to ensure quality 'product', and districts to operate in (generally near docks or city gates) to manage the noise, filth, and brawls over women that were inherent to the whole business. The 'trade' was also clearly not small, much less a small part of life in the world early Christianity was addressing. While it is very unclear what the exact percentage of women could be described as pornēs would be in any western society before the advent of the modern census, it is clear that at the time it was at least astonishingly large - particularly after military victories against foreigners when cities were flooded with more cheap pornēs than they could rape at any price. It is also important to consider that every woman in that era had the threat of being sold into porneia hanging over her head, as women who lost the social status granted to them by a man for whatever reason could always be sold or abducted for 'scrap value.' This would have been true to varying degrees whether that status was by virtue of being somewhere on the sexual-partner-to-a-man spectrum from 'wedded wife according to the laws,' kept as part of a relationship with her father's family and for the purpose of producing heirs, to disposable girlfriend to sexual chattel or by virtue of being maintained as a daughter or sister or cousin. Losing that connection through shifting political winds or sexual disinterest or familial indifference or military defeat could mean losing everything. Demeas in Menander's play Samia describes, in detail that would make the vilest MRA blush, what will happen to his companion (hetaera) when he kicks her out of the house for supposed adultery:

"You think you're so fine. Go to the city and you will see what kind of woman you really are. They live in a different world those other women, paid a paltry ten drachmas for running to dinner parties and drinking neat wine until they die, and if they hesitate or demure, they starve. You will learn the hard way like everyone else, and recognize the mistake you have made."

Indeed a comic character later expounds on this idea:

"Apart from that its easier, isn't it, to get along with a 'married' hetaera than with a wedded wife. Of course it is. A wife stays indoors, her haughtiness licensed by law, a heteaera, on the other hand, knows that if she wants to keep her man she must pay for him with good behavior, or go and find another one."

Women had reason to fear the life of a pornēs in a kinētērion (Classically translated as fuckery or fuck factory), according to Eupolis 99.27,

"They stand virtually naked, lest you be deceived; take a look at everything. Perhaps you are not feeling up to the mark; maybe you have something on your mind. The door's wide open; one obol's the fee. Pop in! No coyness here, no nonsense, no running away, but without delay the one you want, whichever way you want her. You come out; you tell her where to go; to you she is nothing."

can anybody here post one completely original thought regarding God? Like something that isn't taken out of the bible or what other people have written about that book? I used to believe in God but now I just think its all man-made BS anybody please...just share one original thought about who God is, or how we got here or why we exist. EDITED: for typos