If a guest requests not to sit with a single-rider. The CM has the option of doing or not doing so. What is so different with this than requesting for a row you want to sit in? Which again, the cast member has the option of doing so or not doing so. It has nothing to do with slowing down the lines. So that is out of the question.
If a group doesn't want to ride next to a single rider. They won't. This is a simple solution that is being blown over-board.