On the 15th day of the Mahabharata war,the illustrious Dronacharya was instigated by Duryodhana's remarks of being a traitor. Dronacharya used the Brahmasthra against the ordinary Pandava kshatriyas. At that
moment, all the Sapta Ṛiṣhis appeared on the sky and asked Dronacharya to retract this ultimate weapon used on ordinary kshatriyas. Dronacharya obeyed the command of the Sapta Rishis, and retracted the divine weapon.
It became evident that the illustrious Dronacharya would singlehandedly wipe out the Pandavas, as Arjuna, conversant with the ways of Morality, desisted from fighting Dronacharya, when he was fighting the Panchalas.
Of pure souls and pure conduct, O king, and keeping heaven in view, they fought according to righteous methods.
Advice given by Lord Sri Krishna
.At this point in time, Lord Sri Krishna gave the following advice to Yudhishthira(king of the Pandavas in battle):-
'If Drona fighteth, filled with rage, for even half-a-day, I tell thee truly, thy army will then be annihilated. Save us, then, from Drona. Under such circumstances, falsehood is better than truth. By telling an untruth for saving a life, one is not touched by sin. There is no sin in untruth spoken unto women, or in marriages, or for saving king, or for rescuing a Brahmana.
Why women and marriages are covered in the advice
During the Swayamvara of Draupadi, the Pandava princes(who were in exile then) had gone to the Swayamvara as Brahmanas. An untruth was spoken there by Arjuna, to avoid a battle and prevent loss of lives of Brahmanas, who were supporting him(this is allowed as per [Manusmriti, Chapter VIII]-rule 104) and Lord Sri Krishna who was present there recognised that the youth in the guise of a Brahmana, was infact Arjuna:-
Then hearing those words of his, Phalguna replied, saying, 'O Karna, I am neither the science of arms (personified), nor Rama endued with superhuman powers. I am only a Brahmana who is the foremost of all warriors and all wielders of weapons. By the grace of my preceptor I have become accomplished in the Brahma and the Paurandara weapons. I am here to vanquish thee in battle. Therefore, O hero, wait a little.'
So, Lord Sri Krishna , in an implicit manner, was probably informing the reluctant Yudhishthira, that the Pandava princes, had already faced a similar situation before, where an untruth was spoken to save lives(during Princess Draupadi’s Swayamvara).
There is no sin in untruth spoken unto women, or in marriages, or for saving king, or for rescuing a Brahmana.
It is a fact that untruth was spoken during the Swayamvara of princess Draupadi.......and women, marriage and rescuing brahmanas...all were relevant and related to that event.
This appears to be one of the probable reasons, why-marriage,women and rescuing Brahmanas, have all been covered in the same breath,in the advice given by Lord Sri Krishna.