There's nothing unusually scandalous about it compared to arranged marriages from medieval Earth, no, though as I mentioned earlier arranged marriages are not something that are usually practiced by people in the spotlight in Rodoria (which, despite having failed in many regards and having occasional relapses, has endeavored to promote gender equality). The reason for the marriage is simply to cement an alliance between Zerul and Pelgaid, (since Pelgaid recognizes that it is not only the easiest target out of the ten duchies, but also that they cannot risk standing alone if the Black Tribunal decided to seize the opportunity to make a concerted effort to break out of the Land of Eternal Darkness while the civil war ensures that Pelgaid has no reinforcements; Zerul, meanwhile, ensures that Etlon can't move through the otherwise passive Pelgaid to attack them) though this does raise a number of new political issues, most notably that with the Blue Duke's only heir being married to the White Duke and living in Pelgaid, succession is going to be a big unknown.
That aside, the main issue with it is that the couple to be wed have had very little interaction with one another and are being married despite not knowing each other very well. There's no unusual difference in age between the two of them (Duke Frederick Pelgaid being twenty-four years old and Selene Zerul being twenty-one), there are no money changing hands (aside from those spent to make a spectacle of the wedding) and neither party is technically being forced to agree to the marriage by anything other than the political conditions surrounding their separate duchies and the related duties and expectations.
The reason most monarchs would have protested would in part be the questions of succession being raised in Zerul, and in part simply that it's an arranged marriage between two people who hardly know each other (and are sufficiently high-profile for them to take notice).