When you bring up the context menu and select Arrange Marriage on someone outside of your court, the game does not set that character as the target for the arrange marriage interaction, it only establishes the liege of that person as recipient of the interaction. Therefore not only do you have to select this person AGAIN as target, you also have to wade through all the people in your court of the wrong sex to find the person you wanted to arrange the marriage since there is no target to filter on.