I agree that every person, whether coupled or single, has the same opportunity to book their preferred seat ahead of time. Therefore it can not be rude for couples who book first to choose what they prefer. Yes it is rude to talk around someone, but that is a seperate issue. That is what it rude, not the booking of preferred seats for whatever reason. Someone's thought process can't be rude, only their actions. Booking two particular seats is not rude. The rude things would be: not taking no for an answer when offering a switch, sighing or acting otherwise annoyed when the stranger refuses, or talking over a stranger the whole flight. If the couple just sits there and does their own thing the whole flight, I can't see how this could inconvenience the single traveler in any way, any differently than sitting next to two unrelated strangers would. So their internal motivation makes no difference as long as they don't act rudely.