in Gen 29, he “marries” one girl (which likely involves a ceremony and vows) but has sex with another and is thusly married to her

also in Genesis, it says “he went in unto her, and she became his wife”

only other definition of marriage given other than ”between a man and woman” is when a man leaves his parents and cleaves unto his wife and they become one flesh

Jesus reaffirms this last definition and if He is reaffirming Genesis on marriage chances are the rest of Genesis on marriage counts too, none of it conflicts

all mentions of ”fornication” are in English translations and to the best of my knowledge the Hebrew and Greek best translates to sexual immorality, and the definition of fornication has actually changed in the English language in recent years to its current meaning

10 commandments talks about adultery not ”premarital sex” (no such thing), and in Deuteronomy (I‘m pretty sure, it’s one of the Mosaic books at least) talks about how if a man and woman have sex they should be considered married and not depart from one another