In the Book 1: Adi Parva, Sambhava Parva (Section 104) of the Mahābhārata, we encounter an appalling story of sage Dīrghatamas, as narrated by Pitāmaha Bhīṣhma to Satyavatī.
"There was in olden days a wise Rishi of the name of Utathya. He had a wife of the name Mamata whom he dearly loved. One day Utathya's younger brother Brihaspati, the priest of the celestials, endued with great energy, approached Mamata. The latter, however, told her husband's younger brother (i.e., brother in law) — that foremost of eloquent men—that she had conceived from her connection with his elder brother and that, therefore, he should not then seek for the consummation of his wishes. She continued, 'O illustrious Brihaspati, the child that I have conceived has studied in his mother's womb the Vedas with the six Angas, Seed is not lost in vain. How can then this womb of mine afford room for two children at a time? Therefore, it behoveth thee not to seek for the consummation of thy desire at such a time. Thus addressed by her, Brihaspati, though possessed of great wisdom, could not suppress his desire. The child in the womb protested, 'There is no space here for two. O illustrious one, the room is small. I have occupied it first. It behoveth thee not to afflict me.' But Brihaspati without listening to what that child in the womb said, sought the embraces of Mamata possessing the most beautiful pair of eyes. And the illustrious Brihaspati, beholding this, became indignant, and reproached Utathya's child and cursed him, saying, 'Because thou hast spoken to me in the way thou hast at a time of pleasure that is sought after by all creatures, perpetual darkness shall overtake thee.' And from this curse of the illustrious Brihaspati, Utathya's child who was equal unto Brihaspati in energy, was born blind and came to be called Dīrghatamas (enveloped in perpetual darkness). And the wise Dīrghatamas, possessed of a knowledge of the Vedas, though born blind, succeeded yet by virtue of his learning, in obtaining for a wife a young and handsome Brahmana maiden of the name of Pradweshi. And having married her, the illustrious Dīrghatamas, for the expansion of Utathya's race, begat upon her several children with Gautama Dīrghatamas as their eldest.
Now, it's needless to say what kind of acts does these verses talk about.
I'm seeking an explanation for these unfathomable act committed by Devaguru Bṛhaspati. Considering, he's is supposed to be the all knower of all kinds of dharmas, why must he commit such unpardonable activities?
Manu Smṛti says the wife of one's elder brother is to him like the wife of his Guru.
However, here we see Bṛhaspati, all lost in lust trying to gain sexual pleasure with his brother's wife (who's by the way pregnant). How is this even justified from any perspective?
Further, on being refused by the unborn foetus (i.e., Dīrghatamas), the "passage" to commit this heinous act, Bṛhaspati curses an unborn innocent child with perpetual darkness (blindness). How can such an exalted being be so much lost in materialism to give an unnecessary and unjust curse?
I'm unable to reconcile anything w.r.t. Bṛhaspati's acts. Please explain in detail if these acts are justified or not when viewed from the dhārmika perspectives. Has any explanation been given for this appalling story in any other scripture or by any "authoritative personality"?