"But when evening came, he took his daughter Leah and brought her to Jacob, and Jacob made love to her. And Laban gave his servant Zilpah to his daughter as her attendant." NIV translation
Jacob was a schemer but he met his match with laban. At the wedding feast, the bride would have been veiled and so Jacob thought that he was getting Rachel but Laban switched and gave him Leah. Jacob slept with her and consummated the marriage before he knew what was happening.
"When morning came, there was Leah! So Jacob said to Laban, 'What is this you have done to me? I served you for Rachel, didn't I? Why have you deceived me?' Laban replied, 'It is not our custom here to give the younger daughter in marriage before the older one. Finish this daughter's bridal week; then we will give you the younger one also, in return for another seven years of work.'" NIV translation
When Jacob realized what had happened, he confronted Laban about the deception. Laban's answer was to use their tradition as an excuse for his lie. His solution to the problem was to give Jacob a second wife in exchange for seven more years of work.
In this we see just how far Jacob has strayed from God and His ways. He must have known that God's plan was for a man to have only one wife as he had grown up in that environment and had probably heard of his grandfather's mistake with Hagar.
But, Jacob was going with the flesh as he really wanted Rachel and Laban was not even going to make him wait another seven years. It was a big deal back then to be in debt to someone and especially for a long period of time. This is the way that it often goes when you step outside of God's perfect will for your life. It starts with a little compromise and, before you know it, you are feeling isolated and a long way from God.
We also see the dangers of elevating the traditions of men to a position of equality with the words of God. Jacob did not give the matter a second thought when he heard that their custom would allow him to have a second wife.