Do I need to quote the Old Testament verses that show that men had multiple wives back in Biblical times? I think it is fairly obvious.
Hillel and Shammai were
divided on divorce. They each based their approach on the way they interpreted a verse in the Bible:
MISHNAH. BETH SHAMMAI SAY: A MAN SHOULD NOT DIVORCE HIS WIFE UNLESS HE HAS FOUND HER GUILTY OF SOME UNSEEMLY CONDUCT, AS IT SAYS, BECAUSE HE HATH FOUND SOME UNSEEMLY THING1 IN HER.2 BETH HILLEL, HOWEVER, SAY [THAT HE MAY DIVORCE HER] EVEN IF SHE HAS MERELY SPOILT HIS FOOD,3 SINCE IT SAYS,4 BECAUSE HE HATH FOUND SOME UNSEEMLY THING IN HER.5 R. AKIBA SAYS, [HE MAY DIVORCE HER] EVEN IF HE FINDS ANOTHER WOMAN MORE BEAUTIFUL THAN SHE IS, AS IT SAYS, IT COMETH TO PASS, IF SHE FIND NO FAVOUR IN HIS EYES.6
"Unseemly thing" in this context is sexual misbehavior.
If Jesus said that a man commits adultery if he marries another woman after a divorce, then he he is disagreeing with Mosaic law. He in fact says as much in Matthew. He has a more idealistic view of what marriage should be than what existed in Old Testament times. Multiple wives were outlawed in Judaism much later, I think 10th century (Rabbeinu Gershon), and even then the ruling was not accepted by all communities.