I want to point out that the proper boundaries of sexual morality can and must change significantly depending on other parameters of a given society.
To the best of my understanding, historically there has been a lot of concern regarding rights of inheritance. The chances of survival and of material confort of any given person had a lot to do with whether that person had been acknowledged as the lawful inheritor of the wealth of someone else. To this day, a major reason for marriage to exist at all is to establish financial commitment between the spouses and, to a slightly lesser degree, a responsibility of the spouse's families towards the other spouse's well being.
That is why provisions for allowing men to marry the widows of their own brothers, which sound so odd today, are so recurrent in the Abrahamic religions. There was a very significant practical result of those. Those societies did not really have much of a reason to even state that men are supposed to earn the material means for sustaining their own wives and children, while the wives are expected to seek a honorable man that chooses to provide for them and their offspring.
That was simply how things worked back then; people could not reasonably be expected to feel any significant drive to put their hard effort into seeking the means for safety and economic safety for those not of their own family.
In such an environment, it was very much necessary to take steps to assure that people had some certainty of who their parents and children were; that their wives did not have intercourse with other men; that their men were capable and willing of providing material means for their wives and children. And, significantly, that adultery was perceived (fairly) as not too different from continued stealing of other people's honor and material rights.
It is an entirely different situation when a society is willing to accept that women can earn their own livings and some sort of provision must be made for men that are actually unable or unwilling to provide proper material support for their own children and wives. All the more so when effective birth control is also available, and there are various ways of people for earning material safety without it necessarily coming from their own biological or adoptive parents.
The priorities and even actual needs jump all over the place, and not all of the results are reassuring. There is a lot more freedom, but also a lot more complexity and uncertainty.
Still, it means that fornication loses so much of its drawbacks that t may easily become a good thing overall, and adultery becomes a very lesser concern, or even no concern at all if a proper understanding and structure are in place for a given specific couple.
To a degree I sympathise with the concerns that I think I see in many Muslims, particularly those that do live in patriarcal societies similar to those of the time of the Abrahamic scriptures. Besides the plain fact that their expectations make sense for the parameters that they know and expect to live under, there is also the real and often neglected concern of what to do with those who fall under the cracks of all those complex and varied situations.
I know how troubled I feel when I realize that certain people are actually allowed to raise children despite what to me looks like complete, even criminal lack of proper means (not always financial means at that). There is a considerable degree of voluntary delusion and negligence in what passes for consideration for the future of our society. I have seen children be raised into parameters that, to me, look all the world like they were designed to purposefully make delinquents and social parasites out of them.
Still, those challenges must be faced in ways other than just longing for simpler, more predictable times.