"In the morning, behold she was Leah!" Surely she was Leah during the night as well?
Rashi: Of course she was! At night it seemed that she was Rachel, for Yakov had given to Rachel Simanim 1 , and Rachel divulged them to Leah.
According to the Midrash, the Simanim were the three Mitzvos for which women [who transgress them] are prone to die when giving birth - Nidah, Chalah and Ner Shabbos.
How did Lavan succeed in swindling Yakov?
Yakov had given to Rachel Simanim. He could hardly have known that the righteous Rachel would divulge the Simanim. (EC) Refer to 29:12:1:1**.
Ohr ha'Chayim: Yakov was careful not to look at her before Bi'ah, lest amidst desire drops of semen exude prematurely. Also, he extinguished the lights before intimacy 1 , like is proper (OC 240:11)
Moshav Zekenim (49:3) brought that there was no need for a lamp in Rachel's tent, due to her [radiating] beauty. How could he think that Leah is Rachel? Perhaps Yakov did not yet know that Rachel radiated; he had seen her only in daylight, or before she radiated (refer to 29:25:153:3). Tosfos ha'Shalem (28:8) says that Leah was more beautiful than Rachel, but Moshav Zekenim disagrees. (PF)
How was Leah's Kidushin valid? It was mistaken!
Riva: Yakov suspected that Lavan will give Leah in place of Rachel. He intended, if this is Leah, I am Mekadesh her, lest my Bi'ah be Zenus (outside of marriage).
Tosfos ha'Shalem (1): Kidushin through Bi'ah takes effect even mistakenly.
Chizkuni: After he realized that it was Leah, he had Bi'ah with her again for the sake of Kidushin.
Ner Uziel (p.174): Yakov intended for his Zivug. Since he received the Brachos appropriate for Esav, now Leah, the appropriate Zivug for Esav 1 , is also a proper Zivug for Yakov. The physical identity of his Zivug was a trifling detail, so the Kidushin was not mistaken.
Why did Leah trick Yakov?
Da'as Zekenim, from Bereishis Rabah 70:19: She told him, your father called "Esav", and you answered him. So you called "Rachel", and I answered you! 1 From then, Yakov began to hate her.
Why didn't Yakov realize through her voice that it is Leah? He was in the house for seven years!
Riva: He was very modest, and did not talk much with his wife (after the Nisu'in).
Bechor Shor: He had not talked much with Rachel during the seven years.
Moshav Zekenim (14): After a month, he separated from Lavan.
If Yakov thought that he was having relations with someone else (one of the nine Midos of despised relations), Reuven, who was conceived from this, would be spiritually blemished!
Moshav Zekenim, Ra'avad (in Shitah Mekubetzes Nedarim 20b DH Asnas) #1: Reuven was not conceived that night 1 , rather, after Yakov knew that it is Leah.
Ra'avad #2: The blemish is only when he intended for Ervah (someone forbidden to him).
Ra'avad #3: The Torah was not yet given, and they were not forbidden details of Mitzvos. 2
Tosfos ha'Shalem (31:4): Yakov suspected Lavan of trickery. He intended, if this is Leah, I am Mekadesh her through Bi'ah.
Magen Avraham (240:9): The Isur is only when he sees a woman, intends for her, and has Bi'ah with someone else. Yakov had Bi'ah with the body for which he intended.
Birkei Yosef (OC 240:4): The blemish is when one was negligent. Yakov had Simanim; this was Ones.
Yakov's intent was for his Zivug; he had Bi'ah with what he intended for. (Heard from R. Uziel Milevsky.) Refer to 29:25:151:4.
Moshav Zekenim: To resolve this with "Reishis Oni" (Bereishis 49:3; Reuven came from Yakov's first Bi'ah), we must say that Hashem held back Yakov's seed that night. Tosfos ha'Shalem (30:22:2) - Reuven was from the first seed that could father a child, i.e. after the first Bi'ah. The verses connote that after Yakov married Rachel and loved her more than Leah, Leah became pregnant.
Since there was no Aveirah, the child is not blemished. (PF)
Ohr ha'Chayim (23): Due to his mistaken intent, Reuven did not receive kingship, Kehunah and the double portion of a Bechor.
Ohr ha'Chayim #1: Had Lavan given Leah in place of Rachel openly, by force, Yakov would have one complaint. Now that he schemed to do so covertly, this also caused spiritual losses due to Yakov's mistaken intent during intimacy 1 (refer to 29:25:154:5 and the note there). Tzadikim are overly adamant about what pertains to spirituality.
Malbim: Why did you not give to me Rachel, and why did you give to me Leah, whom I do not desire?
Ha'Emek Davar: Why did you not give to me Rachel, and why did you trick me and cause me to have Bi'ah with a woman without my knowledge
Ohr ha'Chayim: Alternatively, Yakov had decided to keep Leah. Because he married her via trickery, this decreased his love for her. Or, Yakov decided that he wanted to marry also Rachel, and feared lest Lavan demand more than seven years work for her. Therefore, he said 'why did you trick me'? I would have agreed to marry Leah. Both of them are equally good (Rachel is not worth more
QUESTIONS ON RASHI
Rashi writes that Rachel gave the Simanim to Leah. Why did she help her father to trick Yakov?
Rashi: It was to spare her sister from shame. (If she does not know the Simanim, Yakov will say 'I do not want to marry this woman!')
Ner Uziel (p.178): Rachel understood through Ru'ach ha'Kodesh that also Leah was Yakov's Zivug.