John Calvin Commentary Genesis 31:50

"If thou shalt afflict my daughters, and if thou shalt take wives besides my daughters, no man is with us; see, God is witness betwixt me and thee." — Genesis 31:50 (ASV)

If thou shalt take other wives besides my daughters. Laban declares that it would be a type of betrayal if Jacob were to take any other wives for himself.

But Laban himself had compelled Jacob to polygamy. For how else did the holy man come to have more wives than one, if not because Leah had been craftily substituted in the place of Rachel? But now, from a pure natural feeling, he condemns the fault of which, blinded by greed, he himself had wickedly been the author.

And certainly, when the bond of marriage is broken, than which nothing among humankind is more sacred, the whole of human society sinks into decay. Therefore, those fanatical people who, today, delight in defending polygamy need no other judge than Laban.