Or HaChaim on Genesis 29:23/אור החיים על בראשית כ״ט:כ״ג

1 ויהי בערב, It was in the evening, etc. Laban caused great pain by deceiving such a righteous man as Jacob; the exchange resulted in a dilution of the hold holiness exerted on earth. You are well advised to study what the Zohar סתרי תורה on Parshat Vayetze item 176 has to say on the subject. According to the Zohar, Laban's having switched Leah for Rachel was the cause that the sanctity of the birthright became downgraded. Originally both monarchy and priesthood would have been the share of the firstborn just as the firstborn receives a double portion of the inheritance from his father. When Jacob slept with Leah he intended to impregnate Rachel (since he was not aware of his partner's real identity). This meant that his marital relations with Leah at the time were not totally pure. As a result the sanctity that he could have achieved by this union if his partner had truly been his intended bride, the sanctity he hoped to create, was diluted, was significantly reduced.

2ויקח את לאה, he took Leah. Perhaps Leah did not intend to deceive Jacob but was forced by her father Laban to go through with this charade. It is also possible that Laban managed to persuade her. Unless one of these two reasons is correct the Torah did not need to write "he took," but could have simply written: "he brought his daughter Leah to him."

3ויבא אותה, he brought her. Laban was careful to do this personally; this was part of the deception. He did not allow time for the deception to be discovered. It would have been very unbecoming for the bridegroom to have demanded at that moment that Leah (Rachel) identify herself.

4ויבא אליה. He had marital relations with her. The reason that Jacob did not notice that the woman beside him was not Rachel was that a righteous man of his calibre did not engage in actions that would arouse him before performing the commandment of marital intercourse in order to produce children. There was no one who could match Jacob in this kind of piety. The Torah itself testifies in Genesis 49,3 that he was able to say of himself that Reuben was the product of the first time he emitted semen, i.e. ראשית אוני. Look at our commentary on that verse. He joined Leah without engaging in what is called "foreplay" in our parlance. Even after completing intercourse with Leah Jacob did not recognise her since everything took place in darkness as required by halachah (compare אור החיים 240).