Hey! Interesting question here. The Wikipedia article touches on a couple of things that I would like to expand upon such as the number of witnesses required, the punishment, and how one could legally apply zina punishments in practice. I'm going to be drawing most of my information from Wael Hallaq's Sharīʿa: Theory, Practice, and Transformations, a book I highly recommend if you're interested in an academic study of Islamic law.

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First it is important to look at what sharīʾa itself is as a system of law. It isn’t how many modern Westerns might view law: there is isn’t a stark split between the “legal” and the “moral” as they are viewed to be one and the same. This makes Islamic law also all-encompassing. There is little room for a “legal but immoral” action, resulting in the division of all actions into one of 5 categories. The wājib (obligatory), the mandūb (recommended), the mubāḥ (neutral), the makrūh(disapproved), and the harām (forbidden). Every action can be seen to fall somewhere on this scale. However, Islamic law only strictly enforces those which are wājib and haram. The other 3 categories are seen to be rewarded or punished in the next life and not in this.

Zina falls under the haram classification. However, it is also considered one of the ḥudūd crimes, meaning that its punishment is fixed by the sharīʾa. There are not many of these laws – 6 in total if I recall correctly – but they are considered among the worst crimes in Islam. In this group are things such as adultery, blasphemy, highway robbery, and gambling. These are also the crimes that are the hardest to commit. There needs to be infallible proof that the crime was committed; if there is even the slightest of doubts (shubuhāt) then the punishment is intended to be overturned. This becomes extremely important in how zina, a crime that is both harām and one the ḥudūd, is treated.

With this background, I think we can start to discuss adultery itself. The exact theory can differ from school to school, so I’m going to try and give more of a general overview.

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Typically to prove zina you need 4 male witnesses to testify to the act. Each of their testimonies must perfectly match. For example, if 4 men all testify that the act has occurred but they do not agree on the location, the time, or even the color of the clothing that the perpetrators were wearing, then the ḥadd punishment can’t be given. To show the extent of this, I want to give an example from the time of the Caliph Omar, taken from the position paper by the Karama, a group of Muslim women lawyers for human rights on “Zina, Rape, and Islamic Law: An Islamic Legal Analysis of the Rape Laws in Pakistan:

Anything else but the actual act of intercourse/penetration is not acceptable to carry on the hadd. The jurisprudential books report in great details the incident relating to al-Mughirah ibn Shu’bah, which took place during the ruling of Omar the second Khalifah and which is very clear about the requirements of the zina evidence. In brief, we learn that al-Mughirah was accused of zina, three of the witnesses testified against him describing the actual act of intercourse. When the fourth man’s turn came, he reported having seen two people in a suspicious setting. He stated, “I saw a behind going up and down, heavy breathing, and I saw her legs on his shoulders like a donkey’s ears. I do not know beyond that.” Although some might think that this testimony should be more than enough to convict the accused man and woman of zina, it was not. Based on the fact that the fourth testimony was not sufficiently explicit, Khalifah Omar dismissed the zina charge and convicted the three men with false accusation.

In addition to showing how hard such an accusation was to prove, this passage also brings up another deterrant to accusing someone of zina, the risk of false accusation. While zina itself is a ḥadd crime, so is falsely accusing someone of it. While the penalty for the former is 100 lashes or death, the penalty for false accusation is 80 lashes. Both of these punishments come from the Qurʿan itself, where it says:

The [unmarried] woman or [unmarried] man found guilty of sexual intercourse - lash each one of them with a hundred lashes, and do not be taken by pity for them in the religion of Allah, if you should believe in Allah and the Last Day. And let a group of the believers witness their punishment. (Qurʿan 24:2. Sahih International)

and

And those who accuse chaste women and then do not produce four witnesses - lash them with eighty lashes and do not accept from them testimony ever after. And those are the defiantly disobedient. (Qurʿan 24:2. Sahih Internaional)

You’ll notice here that the Qurʿan doesn’t stipulate stoning as the punishment for adultery. Although this belief is held by a number of schools of thought, it is a punishment outlined in the hadith and not within the Qurʿan itself. Also, as a side note, it is interesting to note that this Surah that details this punishment for false accusation was revealed to Muhammad around the time that his preferred wife Ai’sha was accused of adultery with one of his soldiers

Moving on, as you noted in your question, these strict requirements made zina very hard to prove. So was anyone ever actually convicted of it? Short story is yes. I’ll highlight below a couple of examples.

“A man from the tribe of Aslam came to the Messenger -peace be upon him- while he was in the mosque and said to him,: ‘O Messenger of God, I have committed adultery.’ The Messenger turned away from him. The man, then, stepped in front of the Messenger and said,: ‘I committed adultery.’ The Messenger again looked away. The man did the same thing four times. When he confessed four times the Messenger called upon him and asked him,: ‘Are you insane?’ The man said,: ‘No!’ The Messenger then asked him: ‘Were you married (muhsan) when you committed this act?’ to which he said,: ‘Yes!’ Only then did the Prophet order that the man be punished for zina

You’ll note here that the man had to confess four times to actually be punished. Although a single confession is technically enough to show guilt, it is recommended that the confessor should repeat his confession the full four times. The confession also should be extremely clear. There can be no metaphor involved but the act must be plainly described (to ensure that people who had differing definitions of zina didn’t wrongly confess). There is another hadith of Muhammad asking a confessor explicitly “did you penetrate her?”, after asking “Maybe you just kissed, maybe you touched her, or looked…” (al-Bukhari). Once the guilty party confessed and suffered his or her punishment, they were then to be forgiven and accepted back into the community.

There were other ways of trying to protect those accused of this crime as well. I won’t get into all of them here but they included things like lengthening the period of gestation from 9 months to, in one case, upwards of 3 – 5 years. If a divorced woman then becomes pregnant, she has a recourse to claim that the child was from her former husband and not from extra-marital sex.

Of course, there were also those punished who had not confessed to the crime but rather had been successfully accused. This was not common however, and heavily depended on the year and place where the case was brought forward. For instance, the Ottoman Empire would have had different conception of how to punish zina, including the punishment of prostitutes, than would have been held by a rural village in Fatimid Egypt.

As for how Muslim women saw these laws, I’m afraid I can’t give a super in-depth answer. Some early converts (including women) saw the entire institution of monogamous marriage and prohibition of other forms of sexual relationships as upsetting the polygamous practices that abounded in Arabia before the rise of Islam. Leila Ahmed, a famous Muslim feminist author, writes in Women and Gender in Islam that:

After being conquered by Muhammad, the men of Taif complained in taking the oath that zina was necessary to them because they were merchants— in other words, they attached no stigma to the practice. One woman taking the oath said, “Does a free woman commit zina?”— a response construed to mean that she felt any union that a free woman entered into could not be termed zina (44).

I’d also like to note here that Islam’s impact on women was vast. Some things improved their position in society, such as inheritance laws that were at the time rather progressive, while others degraded their position. Many of these repressive traditions arose over the coming 1400 years, not under the time of Muhammad himself (for example, the belief that veiling is mandatory for women in certain societies. There is very little in the Qurʿan itself that would suggest the veil is obligatory. It arose instead from other factors such as hadith and cultural traditions). The influence of new marriage laws and zina punishments was only one facet of the changes women faced.

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I’m running out of space here, so I’m going to wrap up. Overall, zina was rather hard to punish with the requirements for the witnesses being extremely high and false accusation being punished. Of course, punishments did occur, and we cannot expect that the exact, theoretical provisions of the law were always followed, meaning that these standards may have occasionally slipped. Some early Muslim converts may have seen the strict emphasis on monogamy sex within certain defined institutions as strange, but that was a belief held by both men and women.

If you have any questions, please feel free to ask!

Sources

Wael Hallaq, Sharīʿa: Theory, Practice, and Transformations

Layla Ahmed, Women and Gender in Islam

“Zina, Rape, and Islamic Law”, a position paper by Karama: Muslim Women Lawyers for Human Rights