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:
- If husband holds the command on his wife after being defeated
[Bhishma's version of morality]
- Husband loses the command on his wife after being defeated
[Draupadi's version of morality]
Husband doesn't have right to gamble his wife
[Vikarna's version of morality]
They were forced to sit quiet:
- Yudhishtira agreed himself to follow DAsa dharma, if won over by Shakuni
- Bhima was not silent and reacted angrily towards his elder brother
- Arjuna pacified Bhima; because such animosity among brothers and deviation from Dharma, was desired by their enemies
All these is explained in detail at: Were the Pandavas's of submissive nature?
Pati Dharma overridden by DAsa Dharma
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]