וַתְּהִ֨י חַטַּ֧את הַנְּעָרִ֛ים גְּדוֹלָ֥ה מְאֹ֖ד אֶת־פְּנֵ֣י יְהוָ֑ה כִּ֤י נִֽאֲצוּ֙ הָֽאֲנָשִׁ֔ים אֵ֖ת מִנְחַ֥ת יְהוָֽה׃
And the sin of the young men was very great before the LORD; for the men dealt contemptuously with the offering of the LORD.
Metzudat Zion
A matter of mockery, as it says in (Shmuel I, 2:17) "the people disgraced"
Rabbeinu Bahya
וישכב את בלהה, “he slept with Bilhah.” Our sages in Shabbat 55, describe this act as בלבל מצעו של אביו, “he disturbed the couch of his father.” When his father referred to the incident on his deathbed (Genesis 49,4), he too used the same expression, saying אז חללת יצועי עלה, “then you desecrated Him who mounted my couch.” Yaakov did not criticize Reuven for sleeping with Bilhah but for mounting the couch on which his father used to sleep with. her.
You find something similar in Samuel I 2,22 where the sons of the High Priest Eli are accused of sleeping with the women (married women) who performed certain tasks at the entrance of the Tent of Meeting. Eli reprimanded his sons after the rumors had come to his attention. The Talmud in Shabbat there also explains that this verse is not to be taken at face value but that what the sons of Eli did made them subject to such accusations being leveled against them. What is described there as the severe sin of the sons of Eli is the fact that they delayed offering the bird-offerings of women who had either been ritually impure through having given birth or had suffered the disease known as זבה. By delaying these offerings, these women’s husbands were effectively prevented from sleeping with their wives until the matter of the offerings had been taken care of. The prophet Samuel accounts this for the sons of Eli as if they had prevented these husbands from sleeping with their wives because they themselves had slept with these wives.
You find something similar in Samuel I 2,22 where the sons of the High Priest Eli are accused of sleeping with the women (married women) who performed certain tasks at the entrance of the Tent of Meeting. Eli reprimanded his sons after the rumors had come to his attention. The Talmud in Shabbat there also explains that this verse is not to be taken at face value but that what the sons of Eli did made them subject to such accusations being leveled against them. What is described there as the severe sin of the sons of Eli is the fact that they delayed offering the bird-offerings of women who had either been ritually impure through having given birth or had suffered the disease known as זבה. By delaying these offerings, these women’s husbands were effectively prevented from sleeping with their wives until the matter of the offerings had been taken care of. The prophet Samuel accounts this for the sons of Eli as if they had prevented these husbands from sleeping with their wives because they themselves had slept with these wives.