Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
You have to know, dear brother, may Allah bless you, that being dutiful to one's parent is one of the most definite obligations. This obligation has been stressed by the Book of Allah, the Sunnah of His Prophet (Sallallahu Alaihi wa Sallam) and the consensus of the Muslim Ummah.
Marrying a second wife is not Wajib (obligatory) if one is content with his first wife.
Therefore, you have to put the obedience to your parents in priority and try to convince them with what you wish. If they agree then that's good, otherwise do not disobey them as long as it does not cause any forbidden matter.
Anyway, we advise the parents to fear Allah in dealing with their children and not to forbid them from doing what Allah, The Most High, has made permissible for them or ordained for them to do.
The parents should not exploit the obligation of the children to obey and be dutiful to them, as a way to oppress and constrain them. May Allah have mercy on the parent who helps his son or daughter to be dutiful to him (to his parents).