Christians only please, about adultery...?

People say, its ok to remarry, because you got a divorce, your not married any more. But God says, marriage is a covenant, they are bound together as long as he/she lives. a woman who gets remarried after she divorces her husband commits adultery! My point is any remarriage while the previous spouse is still living is adulterous. If they got divorced in the first marriage, how could the 2nd marriage be considered adultery?