BIBLE SKILLS: Use the clear passages to understand the less clear ones:
The clear verses: there is no argument about what these next two passages mean:
Mark 10:11-12 (apostolicbible.com)
"Whoever divorces his wife and marries another commits adultery (moichátai) against her.
And
if a woman divorces her husband and marries another, she commits adultery (moichátai)."
Luke 16:18 (apostolicbible.com)

"Whoever divorces his wife and marries another commits adultery (moicheuei);
and
whoever marries her who is divorced from her husband commits adultery (moicheuei)."
The clear passages: there is no argument about what these above two passages mean:
Sexual relationships outside of that lawful, first marriage are adultery
because God considers that first couple to still be joined as man&wife:
Man cannot change the fact that God has joined together that first-wife/first-husband as one flesh.
(Nor can these passages be divorced from their contexts, Mark 10:2-10 & Matthew 19:3-10).
BIBLE SKILLS: Use the clear passages to understand the less clear ones:
Since its rise in the 1500’s, Western Protestantism has not been willing to agree on the passages below. The cause of so much of the argument is the phrase, “except for sexual immorality (porneia).” For both of these passages, the underlying Greek for the term “sexual immorality” is “porneia” and is often translated, “fornication.” This is different from “commit adultery” which is “moichátai.”
Matthew 19:9 (apostolicbible.com)

"Whoever divorces his wife, except for sexual immorality (porneia), and marries another, commits adultery (moichátai);
and
whoever marries her who is divorced commits adultery (moichátai)."
Jesus is NOT saying in Matthew 19:9a, “Whoever divorces his wife, except for committing adultery (porneia), and marries another, commits adultery (moichátai).”
Jesus employs different words.
porneia = unlawful sexual relationship: sexual relationship with someone who is not lawfully married to you = fornication.
moichátai = sexual relationship with someone who is lawfully married to someone else = commmit adultery.
The rest of the Word of God also uses these same two words in the same passages, even in lists of sins, showing that the two mean different things.
Matthew 5:32 (apostolicbible.com)

"Whoever divorces his wife for any reason except sexual immorality (porneias) causes her to commit adultery (moichásthai);
and
whoever marries a woman who is divorced commits adultery (moichátai)."
NOR is Jesus saying in Matthew 5:32a, “Whoever divorces his wife for any reason except for committing adultery (porneias) causes her to commit adultery (moichátai).” That would make Jesus to say something inane since it goes without saying that she has already committed adultery aside from her husband.
1 Timothy 6:3-4 NKJV
“If anyone teaches otherwise and does not consent to wholesome words, even the words of our Lord Jesus Christ, and to the doctrine which accords with godliness, he is proud, knowing nothing, but is obsessed with disputes and arguments over words”
Scriptures cited from New King James Version unless otherwise noted.