When Yaakov was tricked into marrying Leah, whom he never wanted to marry, why didn't he then divorce her? Moreover, how did Lavan know ahead of time that he wouldn't divorce her (because if not, his trickery would have been futile and not worth doing)?
One might be tempted to conclude from this that divorce was impossible, or at least extremely frowned upon, before the giving of the Torah. The Mishna discusses (Gittin 9:10) whether divorce can be "no-fault" or should only come as a result of adultery. If the latter was the case, at least before the giving of the Torah, then Yaakov might have been stuck with Leah, lest he severely dishonor both of them by implying that adultery had taken place. And if divorce was nonexistent, then Yaakov would have been even more stuck.
There is however a verse which might indicate otherwise, that divorce was acceptable at the time.
"Esav saw that the daughters of Canaan were displeasing to Yitzchak his father, so Esav went to Yishmael, and married Machalat daughter of Yishmael Avraham's son, the sister of Nevayot, in addition to his [existing] wives." (28:8-9)
What is the point of saying "in addition to his wives"? We already know he has wives (they were the motivation for Yitzchak's displeasure), so what does this phrase add to the verse?
It could just be redundant, since we know that genealogies and similar passages in the Torah do often include redundancy, and this passage is in a relatively similar style.
But it is also possible that the phrase is teaching something new: that Esav could and should have divorced his Canaanite wives, but chose not to. Thematically, this seems likely: the influence of Canaanite idolatry in the household would not be removed by marrying a righteous wife, but by removing the evil ones. At the same time, divorcing a wife whom you still love is no easy task. Esav recognized the problem but was not willing to sacrifice in order to solve it. Rather, he took the easy way out and applied a band-aid solution, marrying a new wife instead of divorcing the old ones.
If the latter approach is correct, we must ask why Yaakov too did not divorce his wife when he had the option. It seems to me that the fact Yaakov chose this way, and Lavan knew he would chose that way, indicates something positive about Yaakov's character.
In the ancient Near East, divorced women would have been at a large disadvantage on the marriage market - they were no longer virgins, and may have suffered the previously mentioned suspicion of past adultery. And Leah was unattractive to start out with. Why would Lavan arrange a trick marriage for Leah unless he despaired of finding a regular marriage? In addition, in a society where men did the farming and trading, a woman of any status had little purely economic value. If Yaakov divorced her, Leah would likely live the rest of her live single, lonely, and resented as a financial burden on her family. It is to Yaakov's credit that he spared her this fate, remaining married to her when he had no desire to marry her in the first place.
Indeed, Yaakov's considerations seem to be the original moral justification for allowing polygamy (a justification which, clearly, would not exist in modern society). But while polygamy permitted a man to marry a second wife whom he desired, in a way that was beneficial to both of them, Yaakov's decision was not mutually beneficial. It did nothing for himself, but a great deal for Leah.
Lavan knew ahead of time that Yaakov would probably act this way. He surely felt that he was getting the best of the deal, by taking advantage of Yaakov's foolish piety. But in the historical record, it is he who comes off as beneath contempt, while Yaakov more than succeeds in retaining his moral dignity.