Asked by cynicalclassicist

When Cersei claimed Robert would set her aside what did she mean by that, as that line is used by Renly supporters to show his plan was to make Marg Queen through setting Cersei aside?

It’s not really clear. Certainly there have been cases in Westerosi history in which consummated marriages have been put aside, but none of those carried the same weight as Cersei’s. Prince Daemon Targaryen urged his brother to put aside his, Daemon’s, marriage to Lady Rhea Royce, though what grounds he believed he could do this on remain unclear (perhaps, because the marriage was “barren”, Daemon was prepared to declare that he had never consummated the match? Who knows, and it certainly wasn’t beyond Daemon to make an illegal move to get what he wanted); in any event, it’s worth noting that Viserys refused to do so. Aegon V demanded that Prince Duncan put aside Jenny of Oldstones once he had found out they were secretly married, though they too were childless (and Jenny, as an apparently family-less young peasant woman, had no power of her own to argue against the Iron Throne if the king stated that a marriage between the two had never taken place at all). Tywin managed to have Tyrion’s marriage to Tysha set aside, I think probably by having the septons involved declare that since the officiant was drunk, the witnesses were pigs, and the two married people were underage, theirs was not a valid marriage ceremony and, thus, no true union (again, it certainly did not hurt that Tysha was an orphaned crofter’s daughter and Tywin the Lord of Casterly Rock - he had the power and wealth enough to bribe and/or intimidate Westerosi religious powers into giving the result he wanted, and she conversely had no ability to appeal the judgment).

But Cersei’s case would have been a dramatically different affair. After all, she was a Lannister of the Rock, married to the king for the better part of two decades, and had had three children in that marriage whom the king recognized as his (even if they were not, in fact, his own), including two sons. She could not be removed without all the power of Casterly Rock and its vassals and allies coming to challenge the putting aside of a wife who, at least in terms of appearances, had done nothing to merit her union with the king being dissolved - and that’s likely going to mean war. Even if there was a possibility (on unclear legal/religious grounds) that Cersei herself could be ousted for a new wife, the three Lannister-Baratheon children would still be recognized as the king’s heirs, since declaring them illegitimate would be a dynastically very dangerous next step for him to take. Robert might risk setting Cersei aside for a “new Lyanna”, but he would still have the three children of that union, whose only mark against them was being born to a wife to whom he was no longer married.

Of course, Cersei knows a very valid reason why Robert could not simply put her aside, but have her and her children killed - namely, that she had been sleeping with Jaime for the entirety of her marriage, and that all three of the children were biologically Jaime’s. This is why it is absolutely ludicrous to believe that Renly didn’t know about the incest when he was trying to make Margaery Robert’s queen: he needed a guarantee not simply that Cersei would be put away but also that the three children would have no chance of inheriting. The Tyrells, quite frankly, would not take the risk for anything less than Margaery becoming queen and a Tyrell-bred grandchild assuming the throne one day, and for that, there needs to be far more bolstering Renly’s plan than a handful of weak historical parallels.