I saw that trailer too. As far as the whole Jeyne plot goes, her mother induced her to sleep with him (possibly without Jeyne realizing what she was being encouraged to do, given her actions when Jaime meets her) and encouraged their attraction to gain Tywin Lannister's favor. Once Robb slept with her, he married her in keeping with the moral issues (suggesting Tywin might have known the truth of Jon Snow's parentage - he discusses the union by saying Jeyne was her mother's daughter and Robb was his father's son: yet, Ned very specifically did NOT marry every woman he is believed to have had sex with, nor does it appear that Jeyne is the social climber her mother seems to be).

Given the pandering to 21st century sensibilities through much of the current season, I would have to guess that Jeyne is being dropped altogether. She had no other role in the books beyond being Robb's inadvertant downfall, so she is replacable. Given such idiocy as Dany's indignation over the sexism of the other kahls who killed Rahkaro for following a woman, I am guessing the writers have no idea how to sell a modern audience on the justification for Robb's marital choice vis a vis that honor issue. Instead, they are going to create a love interest who they believe will appeal to the audience more than some vapid, beaten-down Frey girl, and thus cause us to accept/understand his marying her out of love and preference rather than honor.

In truth, the honor thing WOULD be a big deal, even if we (and the books) affect to disdain honor in favor of pragmatism. Honor was not so much an ideal of behavior as a means of verification when there was not legal system. You strive for honor not so you will be admired, but so people know you can be relied upon. You keep your word, because if people cannot trust you to keep your word, they will never ever do anything that relies on you to keep it. When apologizing for his slight to the Freys Robb says that kings must keep their word more than anyone else, not because kings are supposed to be morally superior to others, but because the power of a king depends on the support of reciprocal agreements with his subjects and vassals. If the king will break his promise to marry Walder Frey's daughter, how can anyone else trust him to keep his promises to protect them from their enemies? That's why his bannermen called in their supporters and marched south with Robb to protect his mother's father and his lands - they expected that he would do the same thing for them. That's why the Greatjon respects Robb after the finger-eating incident: his fealty to the Starks and Winterfell is contingent upon their ability to help protect him and his people. By the decisive response Robb (and Grey Wind) give, he shows his mettle, and by his words afterwards, he gives indication of his character: he's not too strict or overly proud or threatened by Greatjon beating his dick a bit, but at the same time, he service notice that he won't tolerate excessive dissent either.

The whole society of Westeros is built upon such personal relationships and the capability of individuals to fulfill their part of an alliance, and honor ties in directly to their willingness to go out of their way to fulfill those alliances and agreements. Robb has to marry Jeyne to reassure his people that he will do what is right. We have seen what happens with a Robert Baratheon or a Rhaegar Targaryen in the ruling House who goes around putting it to women he is not supposed to. It leads to civil war or rebellions or uncertainty about the right to rule. It renders the women unsuited for marriage, since inheritance is a significant factor in the transfer of power. A woman's sexual fidelity is of paramount importance to her reproductive value and the continuity of leadership; it is NOT (or not entirely) about keeping women down or depriving them of rights.

That's also why Renly makes the seemingly gratuitous comment to his host about how his brother (Robert) would have taken advantage of the hospitality and availed himself of the sexual services of his daughters; he is reminding his noble audience that he is not the sort to abuse their allegiance and their forebearance. We get a hint of this stuff with the references to Roberts' defloration of Stannis' wife's kinswoman. She had to give up her son, and was not considered eligible for a good marriage (the fact that Tywin was considering her for Tyrion is proof of that - the emminently practical Tywin would only try for women who were desperate for a spouse), due to her having a bastard. The rank of her husband, and the connections she had did not save her reputation or eligibility and it came down to a choice between a dwarf and a guy who brought her family no advantage. By marrying Jeyne, Robb is proving that he is not that kind of asshole, even if in the general course of things, 98% of the people would forgive him the circumstances. His dishonor was in sleeping with her in the first place, when he had no right to make that choice, but just try to convey that idea to our society, especially on HBO, of all channels.