1- It’s injustice to her if she stipulated in the marriage contract that her husband cannot marry another woman without her permission. But if she did not stipulate such a condition, and he never agreed to such a condition, then technically it’s not injustice if he has a valid reason for it. Society might consider it unjust, but in God’s law it is not.
2- A man must be responsible, protect his family and avoid anything that would end a marriage. If a man pursues such marriage just for pleasure, knowing that this could destroy his family life, he must be responsible and must avoid that. If he causes harm to his family, his marriage ends, and the children suffer, Allah will hold him accountable for that.