As we learn in the Torah and this section of Sefer Ha-Aggadah, Jacob practiced not only plural marriage (since he married two women), but what we would regard as incestuous marriage, since he claimed, in wooing her (falsely) to be Rachel's (and Leah's) uncle ("I am your father's brother), when in fact he was simply their first cousin. (And in fact, first cousin marriage continued to be practiced in Eastern European Jewish families into the last century, and there may be such a marriage in a not-so-distant branch of your very own family tree.)
Mostly, in studying these sections of the Torah, we sort of skip over these marriage practices. We pay close attention to the personal traits of the individuals involved, and our aggadot include Leah's very apropos comeback to Jacob when he says, in effect, "How dare you trick me by answering to someone else's name while I was having sex with you?!?" -- to which she replied, "Who do you think taught me to do THAT, 'Esau'?" -- recalling Jacob passing himself off as Esau in obtaining their father's blessing. (If you ever wondered what the expression, "Hoist by his own petard" means, THIS is what it means.) Certainly a great deal of ink has been spilled about how to understand this idea of tricking one's way into the Divine plan -- Jacob's trickery which somehow brought about God's intended outcome, and Leah's, which did the same. This text raises very pointed questions about whether the ends justify the means, and what a person is allowed to do if he or she is sure the outcome aimed at is the right one.
But standing behind this particular incident, with Rachel and Leah, is a set of marriage and family practices which are undertaken today only by rather reviled sects of people. (That's setting aside that what Rachel and Leah conspire to do is in fact the rape of a man by trickery -- imagine our reaction to a man who blindfolds his girlfriend and then allows another man to have sex with her. At least no one can accuse us of whitewashing the Torah!) What are we to make of that?
For the most robust defenders of plural marriage (who typically are defending "Biblical-style" plural marriage, meaning asymmetric androcentric polygamy), the presence of the practice in the Torah is a sufficient warrant for it (and later prophecies, like Joseph Smith's, "restore" rather than invent the practice). Others of us, taking either a post-Enlightenment view (we've made progress since Toraitic times) or simply a less literal view (we are not meant to live, literally, as the Bible describes), note that slavery, too, was a practice acknowledged in the Torah, and not morally criticized there, yet WE do not practice or endorse it -- in other words, the mere presence of a social arrangement in the Torah is no argument for its moral acceptability, at least not to us.
There is a great deal to be said about monogamy and monotheism -- as social practices, and as spiritual practices, which may or may not support one another. But if the patriarchs are any example, there is clearly no necessary connection between them. Whether this is a device that may be used to interrogate any too-ready assumption that proper spirituality will dictate monolithically the proper arrangement of one's intimate life -- or simply allow us to turn the Torah back on itself, using its own principles to criticize the behavior and "lifestyles" of its leading protagonists, is an open question.