In Mahabharata, when Draupadi was being disrobed by Dushasana, why did the Pandava’s sit quiet? Is there any other reason besides the fact that they had already lost in the gambling and had become slaves?
The primary duty of a husband is to protect his wife. How could Yudhisthira fail to understand this?
This was a situation, where a husband may not have ability to protect his wife, despite of his will. This is applied in all the cases below:
They were forced to sit quiet:
All these is explained in detail at: Were the Pandavas's of submissive nature?
PAndava-s were bound by the morality (Dharma).
As you correctly pointed out, a husband is bound to protect his wife. However, at that crucial moment, unfortunately a slave is not supposed to go against their masters.
This was kind of indirectly admitted by the great Bhishma.
Bhishma said, 'O blessed one, morality is subtle. I therefore am unable to duly decide this point that thou hast put, beholding that on the one hand one that hath no wealth cannot stake the wealth belonging to others, while on the other hand wives are always under the orders and at the disposal of their lords. Yudhishthira can abandon the whole world full of wealth, but he will never sacrifice morality.
[ Why Bhishma did not say anything in Draupadi's Cheerharan? ]
Draupadi didn't doubt what Yudhishthira was following.
while dragged by Dussasana, the modest Krishna consumed with anger, faintly said-- ... The illustrious son of Dharma is now bound by the obligations of morality. Morality, however, is subtle. Those only that are possessed of great clearness of vision can ascertain it. In speech even I am unwilling to admit an atom of fault in my lord forgetting his virtues. [Sabha Parva]
But, to save the situation, Drauapdi herself attempted to distance herself from the dice game played by PAndava-s, & hence indirectly relieving them from the duty of protection.
Draupadi said,--'O son of the Suta race, go, and ask that gambler present in the assembly, whom he hath lost first, himself, or me. Ascertaining this, come hither, and then take me with thee, O son of the Suta race [Sabha Parva]
Which means, if a gambler loses himself as a slave, then he loses rights on his wife. Since Draupadi was not stacked first, Yudhishthira lost her by losing their own independence.
Her question in clearer words was reaffirmed by Vikarna more explicitly:
Vikarna the son of Dhritarashtra said--'Ye kings, answer ye the question that hath been asked by Yajnaseni. ... This son of Pandu, while deeply engaged in one of these vicious acts, urged thereto by deceitful gamblers, made Draupadi a stake. The innocent Draupadi is, besides, the common wife of all the sons of Pandu. And the king, having first lost himself offered her as a stake. [Sabha Parva]
Bhima was not quiet.
He swore to kill Dushasana very brutally by tearing open the breast of Dushasana and drink his life-blood in battle.
"And Bhima said,--Hear these words of mine, ye Kshatriyas of the world. Words such as these were never before uttered by other men, nor will anybody in the future ever utter them. Ye lords of earth, if having spoken these words I do not accomplish them hereafter, let me not obtain the region of my deceased ancestors. Tearing open in battle, by sheer force, the breast of this wretch, this wicked-minded scoundrel of the Bharata race, if I do not drink his life-blood, let me not obtain the region of my ancestors."
