I've been reading Denis de Rougement's Love in the Western World, though I'm taking my sweet time on it of late. One of the arguments he makes is that adultery is the most prominent form of romantic love in medieval lit because love requires obstacles -- or at least literary love requires obstacles to form an interesting narrative arc.
I think he is right to say that the obstacle issue is one reason that love and marriage are so rarely compatible in medieval lit, but surely that can't be the only reason. Love does indeed thrive on obstacles: consider how you feel when separated from your lover because they have to travel, rather than simply leading busy lives. Throwing up obstacles, real or imagined, seems to help romantic love thrive.
I wonder, for example, about domestic dispute calls to the police. I've heard cops complain that they hate domestic dispute calls the most, because as soon as they arrive they are treated as interlopers, even by the person who called them. I suspect that sometimes the caller doesn't simply change her mind, but instead suddenly sees the police as obstacles to love rather than protectors -- and the storyline in her mind changes. It's hard to tell, since the psychology of the domestic dispute is so bitter and complicated.
Nevertheless, the jealous-husband-as-obstacle can't be the only reason for the prominence of adultery, since many other imaginable obstacles exist, e.g. the jealous father, the supernatural guardian (witch, dragon, etc), geographical separation, etc. Actually, it seems to me that in medievalism (depictions of the medieval in the modern world), non-adulterous obstacles are more common. And surely the most famous real-life love story of the Middle Ages, Abelard and Heloise, fits into the category of jealous father (uncle) rather than husband, followed by the obstacle of castration.
The most influential religious understanding of romantic love, too, is entirely within the bounds of marriage. The image of Christ as Bridegroom and the Church as the Bride is central to the Christian understanding of marriage. In this case, love must overcome a rather large obstacle -- death. The narrative demands that the marriage not be consummated (i.e. Pentacost and the founding of the Church) until after the resurrection, that is, after the Groom overcomes the obstacle of death to be united with the Bride. It goes without saying that adultery in this situation is a big no-no ... of all the angry husbands I'd fear, God's got them all beat in the wrath department.
I think, then, that adultery's prominence is partially explained by its role as an obstacle in the romantic narrative, but its primacy over other obstacles must have another explanation. Perhaps romantic love requires a profane element, and the complete violation of a sacrament of marriage fulfills that role.
In an episode of "Futurama," Bender is dating the Planet Express ship. Leela protests to him, "Bender, dating your co-worker and primary mode of transportation is immoral, illogical and a violation of interstellar shipping statute 437-B." Bender replies, "That's what makes it so nasty!" Why is the nastiness of the relationship an advantage in Bender's cartoon mind? I think this gets at the core of adultery -- if we make the profane aspect of it a virtue, as we find in medieval literature, we have no need to justify our behavior, and indeed justification itself detracts from the "romance" aspect.
Gimme good ol' fashioned married love any day.