Why was Le'ah Senu'ah (hated)?
Targum Yonasan, Bechor Shor and Ramban (citing the Redak): Yakov did not really hate her. 1 The Pasuk means that he did not love her as much as Rachel.
Seforno: Because Yakov subsequently saw signs that she was barren, and thought that that is why she agreed to trick him. 2
Ramban: Because she willingly set out to trick him. Even if one were to claim that she was simply fulfilling her father's orders, Ya'akov considered her guilty of not subsequently informing him, or at least of dropping a hint, that she was Le'ah. 3
Tosfos ha'Shalem (30:8): Yakov thought that if he shows love for her, Esav will hate him doubly. 4
Bava Basra 123a: Esav's deeds were hateful to her.
This is evident from the previous Pasuk (see Kli Yakar there).
Even if she became pregnant right away, it is not recognized until about three months (PF). Ha'aros on Birkei Yosef (OC 240:2): Hashem knew that her motivation was because she hated Esav's deeds. After she gave birth, Yakov realized this and ceased to hate her.
This is in keeping with the Midrash which explains how, when Ya'akov saw how Le'ah had tricked him, he wanted to divorce her. What did Hashem do? He opened her womb and allowed her to have children, prompting Ya'akov to say 'How can I divorce the mother of these children?'
Surely this is not a Heter to hate her! Perhaps he loved her, just he was careful not to show this openly. (PF)
Is being hated a reason to open Leah's womb?
Ramban: Indeed it was, since only Hashem knew that she acted the way she did because she genuinely wanted to marry the Tzadik Ya'akov, so He took pity on her. (Refer to 29:31:1:3*). Hence the Pasuk writes "And G-d saw" - He took pity on her so that Ya'akov should not desert her. 3
Abarvenel (Devarim 21:16): Yes - Hashem always favors the hated wife, and the Bechor is from her.
Like we find by a Sotah. If she is innocent, she gives birth to compensate for her disgrace, including suspicion of her. Tosfos ha'Shalem (8) - Yakov refrained from her, amidst concern lest she was intimate with other men, just like she tricked him.
See also Seforno on Pasuk 33.
According to those who explain that Yakov truly hated Leah, it was forbidden to have Bi'ah with her (this is one of the nine Midos of despised relations), and Bnei Leah would be spiritually blemished!
Ran (Nedarim 20b DH Bnei Senu'ah): The Isur is only if he thinks of another woman at the time of Bi'ah.
Yakov did not hide his hatred. (Re'em, in Shitah Mekubetzes Nedarim 20b DH Acharei, says that the Isur is due to "Lo Sisna... bi'Lvavecha."
Ra'avad (Sha'ar ha'Kedushah, cited in Birkei Yosef OC 240:2): At a time of Bi'ah she was pleasing to him (then it is permitted).
Refer to 29:25:154:3.
למה שאר האבות היו עקרים ויעקב לא?