Referencing MAN's position on divorce, not God's.
What therefore God hath joined together, let not man put asunder.
What God put together, man cannot separate.
And he saith unto them, Whosoever shall put away his wife, and marry another, committeth adultery against her. And if a woman shall put away her husband, and be married to another, she committeth adultery.
The source of your confusion is this - it refers to abandonment or separation WITHOUT a bill of divorcement. It's the same principle today - one cannot have multiple spouses.
It's not a difficult concept.
The confusion seems to lie with you, since Mark 10:3 clearly refers to the bill of divorcement. But I'll humor you. If, in your learned opinion, Matthew is actually talking about divorce then can we at least agree that those who divorce for reasons other than infidelity and who later remarry are, in fact, committing adultery against their former spouse?