Researching the topic... please check this out, let me know what you think. This researcher over 200 hours researching divorce, including reading the leading books/scholars on the subject. In the video section below (I bookmarked/hyperlinked it so I starts at the right spot) he gives 5 reasons why he is certain that porneia in Mat 5 & 19 is not referring to premarital sex. He also goes over the 5 most suggested possibilities and speaks to what is the most appropriate definition/application and why. Of course, that whole section is appropriate to listen to, but the '5 reasons why it's not premarital sex' is 57:26 to 1:00:47. The start (setup) to the end of that point is just 5 mins.
Also, below the video it bookmarks/hyperlinks to his research on the passages you note: Mark 10, Matthew 19 & I Corinthians 7. You'd probably enjoy checking those out. Well, of course, I'd strongly recommend the entire video for you. He also does a video where he answers the questions and "push back" of the first video on divorce and remarriage:
Answering Your Questions and Push-Back on My Divorce and Remarriage Teaching - YouTube.
I listened to the section you mentioned. It may be a while before I have a big enough space of time I would dedicate to listen to the whole thing. But I do not think he gave adequate treatment to one of the views he was dealing with, and in that section, he did not address the point I was making either.
He dismissed the idea that it referred to breaking off an engagement. He overlooks something very basic. A couple that is betrothed in Judaism after the husband pays the bride price for virgins is basically married. A divorce would be required after that point. It's like in western culture if the couple had said the words and exchanged the rings in the church service then went and signed the marriage license, but hadn't spent the wedding night together. That's pretty much the stage an 'engaged couple' in their culture would be in.
I am not saying that is my view.
The point I was making is that they are discussing the law of Moses. Under the law of Moses, men could issue divorce certificates to their wives under certain circumstances. The Pharisees wanted to debate those circumstances. Let's back up and consider that. The law of Moses did not allow wives to issue divorce certificates to their husbands. In the passage, Jesus said if a man put away his wife.... except it be fornication.... Let's stop there. It doesn't say a woman put away her husband except it be for fornication.
Women couldn't divorce their husbands. I think Matthew 19 provides a good argument against polygamy. But it is worth noting that there were men in Old Testament times that the Bible indicates were forgiven of sin or righteous before God who were polygamous--Abraham and David. Polygamy was regulated in the Old Testament, but it was only for husbands with more than one wife. A wife could not have more than one husband. If you were a man reading the law of Moses in the polygynous culture before the Babylonian captivity, you probably would have understood adultery to mean a man having sex with a married woman who is not ones wife, not a married man sleeping with an unbetrothed woman. If ancient Israel judges tried to apply the law, how would they treat the case of a married man having sex with an unbetrorthed virgin. Might they not have compelled him to take her as a second wife? And if a married man's brother died without a child, might not the community expect him to take his widowed sister-in-law and raise up seed unto his brother with her? And if he refused because he was monogamous, wouldn't she, if she followed that passage, spit in his face for refusing to take her?
I also think he glides over the fornication before marriage argument for another reason. Those who hold to that view would likely look at the passage in the Old Testament where the man marries a supposed virgin and finds out that she was not a virgin, then goes back to her father to make the accusation. If she is found guilty, she is to be stoned. The fornication could have happened before or during engagement.
I am not convinced that Jesus is referring to something so narrow, but for some of those who held to the traditional view, who'd been reading the other gospels without the exception clause, they would not be concerned with the broad use of fornication in other passages of scripture but see it as a narrow exception. A problem with that is there no real reason to say that it is so narrow. Maybe if they relied on some church tradition, that St. Andrew or one of the other apostles commented on Jesus' words, but I do not know of any statements to that effect.
I do think porneia in Acts 15 is an umbrella term that includes the various types of sexual immorality in Leviticus 18 for which Gentiles were driven out of the land along with the fornication of sex before marriage, which shows up with a similar word in the stoning of the non-virgin bride married off as a virgin passage--playing the harlot in her father's house as it says. (That passage shouldn't be controversial for most Christian couples, except for the menstruation restriction.) In Acts 15, the issue is one of whether Gentiles have to obey the law of Moses. Later Judaism would debate a similar issue, as to whether Gentiles could be righteous without being circumcised, and they realized Gentiles were under the covenant with Noah. I suspect similar theological reasoning was going around a generation or two before Gamaliel II. The apostles and elders in Jerusalem also urged Gentiles to 'abstain from things strangled and from blood' in addition to fornication. The underlying issue may have been what the scriptures required of Gentiles, and what they had was the Old Testament. There was a long list of forbidden sexual activities for which Gentiles were driven out of the land.
I do think fornication includes premarital sex, both from the passage about the non-virgin, and apostolic usage. Paul warns the Corinthians (I Cor. 6) about fornication, warning that the members of Christ should not be members of a harlot. Corinth had pagan temple prostitutes. Some would argue that it means no sex with prostitutes and is limited to that (an idea I have actually encountered). But I Thessalonians 4 says for no man to defraud his brother through fornication, and that God is the avenger of all such. That wording seems more of a fit for the general idea of sex before marriage.
Something to notice about Matthew 19 is that he turns the Pharisees back to the original intention for marriage in Genesis, that two shall be one flesh. They interpreted Moses to say that Moses commanded divorce. Jesus corrected them that Moses prevented divorce. I do not usually care for the NIV that much compared to some other translations, but I do note that in Deuteronomy 24, it's translated to say under X set of circumstances, don't do Y. I mean, if the man is displeased with the wife for a certain category of offense and gives his wife a certificate of divorce and sends her out with a certificate and the second husband divorces her or dies... then she may not marry the first husband again. That would defile the land. It makes the remarriage to the first husband the defiling act without authorizing the divorce. I do not know enough Hebrew to grasp those subtleties, but they may have had Matthew 19 in mind.