"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"(Matt. 5:32)
"And I say to you: whoever divorces his wife, except for sexual immorality, and marries another, commits adultery""(Matt. 19:9)
"10 And in the house the disciples asked him again about this matter. 11 And he said to them, “Whoever divorces his wife and marries another commits adultery against her, 12 and if she divorces her husband and marries another, she commits adultery""(Mk. 10:10-12).
“Everyone who divorces his wife and marries another commits adultery, and he who marries a woman divorced from her husband commits adultery"(Lk. 16:18).
P1: Jesus says that anyone who divorces His wife and marries another apart from sexual immorality commits adultery, and those who marry such a wife commit adultery.
P2: Adultery is sexual intercourse with someone who is not your spouse, and is by definition committed against one's spouse.
C: The adulterous second marriage resulting from someone divorcing their spouse and remarrying another for reasons other than sexual immorality, is therefore adulterous precisely because the antecedent divorce was not a legitimate divorce in God's eyes, since it did not take place on lawful grounds.
C2: If polygamy were legitimate, such adulterous remarriage would not be illegitimate, since it would simply constitute taking another wife for oneself.