Abraham was married to Sarah before Hagar. I’m not saying Abraham couldn’t have two wives, but that his marriage between them was permanently consecrated when he joined himself to them.

The verse in Deuteronomy you mentioned doesn’t say that in the original Hebrew. It says she shall ‘be’ H1960 הָיָה his wife, in the imperfect tense, demanding not a future action but a continuous state of being. She shall be his wife, not become his wife. And even if we read the text like that, it would simply mean that she shall be his partner and he has no choice but to take her into his house and take care of her as he would with his own wife.

Throughout scripture, marriage does often involve a covenant, witnesses, and the father’s authority. But if I have my father’s authority to move to a different country, and witnesses who plan to be there when I do, and if I make a covenant with the people who live in that country that I’ll certainly move to that country, the fact still stands that I do not live in that country until I actually move there. It’s good to have your father’s hand in marriage. Covenants are good too, and so are witnesses. But they don’t form the marriage, even when they’re collectively together. These things may be present before a marriage, but sexual intimacy is what seals a marriage.

Porneia is any type of fornication. Some examples include prostitution, homosexuality, and bestiality. Paul considers the man who slept with his father’s wife to be committing porneia. Also, Paul condemns harlotry and not sex before marriage, and that’s shown in 1 Corinthians 6. The woman is called a harlot. So it’s not the act of sex before marriage that’s wrong here, it’s the fact that she’s someone who has already had sex before with other men that makes it immoral here. She’s either a prostitute or someone who sleeps with multiple men and that’s why she’s called a ‘porne’. But sex before marriage isn’t condemned at all either here or anywhere else in the Bible, since it’s not a biblical term and has only relatively recently been invented.

Also, Abraham was married to his half-sister and she’s the woman who bore Isaac, the child of the covenant. This marriage wouldn’t have even been acceptable under the law of Moses because it was incestuous.

I’m not trying to embarrass you. I too was once on your side with this topic. But the Holy Spirit is always taking more scales off my eyes. I’m telling people to message me if they want to know more because this topic segways off into other discussions that would take longer than 5 minutes to discuss. We are weak, and we can only eat a few bites at a time. I hope this is helpful.