O.k., I think I found the answer. Not sure if this correct, but it's the best I got. Please comment, if you think this is in error.
Whoever marries a woman, who has been divorced, and the old husband is not dead, that is adultery. Whoever divorces a woman for reasons other than sexual immorality, causes her to commit adultery, but if she is divorced due to adultery, than she is already an adulterer, so the divorcer isn't causing her to commit adultery, because she is already an adulterer. I would assume the opposite is true regarding husbands being divorced by their wives.
Jesus answers the question, why did Moses command one to give a certificate of divorce? It's because you are sinners, so God allowed an out, but it was never the original plan. Just like God loves the world, but many through their ways have fornicated with other gods (worshipping idols), and made God divorce the people HE loved. We have to remember, God is the center piece here, not us sinners.
Anyways, I think God is allowing divorce, because we are sinners, but just because God allows us to sin, doesn't mean it's right to sin. Deuteronmy 24:2 doesn't say that the 2nd marriage isn't adultery, it says,
"2 And when she is departed out of his house, she may go and be another man's wife."
So, why did the man divorce her in verse 24? It's says, "because he hath found some uncleanness in her". What is the uncleanness? She was committing adultery with her neighbor!! So, she already doesn't care about the law, Don't commit adultery...Right?
Jesus than confirms what Deuteronomy 24 says, by Luke 16:18 which says... 18 Whosoever putteth away his wife, and marrieth another, committeth adultery: and whosoever marrieth her that is put away from her husband committeth adultery. Jesus is being a good rabbi, by confirming the Torah.
Why would you put away your wife? Because she is committing adultery. If the wife who is divorced by reasons of adultery, marries another, because of adultery, not sure how one could argue that remarriage o.k. for reasons of adultery. Does that makes sense?
https://www.biblegateway.com/passage/?search=Luke+16:18&version=KJV
Deuteronomy 24 King James Version
24 When a man hath taken a wife, and married her, and it come to pass that she find no favour in his eyes, because he hath found some uncleanness in her: then let him write her a bill of divorcement, and give it in her hand, and send her out of his house.
2 And when she is departed out of his house, she may go and be another man's wife.
3 And if the latter husband hate her, and write her a bill of divorcement, and giveth it in her hand, and sendeth her out of his house; or if the latter husband die, which took her to be his wife;
4 Her former husband, which sent her away, may not take her again to be his wife, after that she is defiled; for that is abomination before the Lord: and thou shalt not cause the land to sin, which the Lord thy God giveth thee for an inheritance.
https://www.biblegateway.com/passage/?search=Deuteronomy 24&version=KJV
I read this commentary down here, think it helps.
Matthew 19 has more on this conversation about divorce:
3And Pharisees came up to him and tested him by asking, “Is it lawful to divorce one’s wife for any cause?” 4He answered, “Have you not read that he who created them from the beginning made them male and female, 5and said, ‘Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh’? 6So they are no longer two but one flesh. What therefore God has joined together, let not man separate.”
7They said to him, “Why then did Moses command one to give a certificate of divorce and to send her away?” 8He said to them, “Because of your hardness of heart Moses allowed you to divorce your wives, but from the beginning it was not so. 9 And I say to you: whoever divorces his wife, except for sexual immorality, and marries another, commits adultery.”[a]
This is no doubt why some couples separate in situations where a serious issue other than adultery is involved.
MacArthur explains the Old Testament passages to which Jesus referred:
Jesus here is referring back to that Deuteronomic law in Deuteronomy 24 in which there are no exceptions. He’s simply reiterating that law but that has to be taken in comparison with a couple of other passages. Since
God in His common grace had allowed the death penalty for adultery to disappear, and it is a kind of common grace; since God graciously had allowed the nations to go their own way sinfully and not punish adultery with death, there was a provision for divorce under one condition …
Jesus is saying this is taking it all the way back to the original law with the one exception that if there is the cause of immorality, unchastity, sexual sin, then there is a granting of the right to a divorce.
…
the death penalty not being enforced, even back in Moses’ day, there was a concession that you who have been offended by immoral conduct of a spouse can divorce …
We find the same statement in
Matthew 5:31-32:
Divorce
31 “It was also said, ‘Whoever divorces his wife, let him give her a certificate of divorce.’ 32 But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery.
https://churchmousec.wordpress.com/2014/08/30/forbidden-bible-verses-luke-1618/