I agree - not rude to book and hope, provided that hope doesn't transmute to expectation.
Once had it happen that I was the middle seat between such a couple. He, with an aisle seat, asked if I would take his girlfriend's window seat. I declined to take the window, as I prefer not to see outside the plane (often triggers nervousness about flying, for me) but said I would trade for his aisle seat. He eventually agreed, didn't seem happy, but it was his choice to book seats not together, and risk having a middle seat occupier who doesn't like the window seat - so he could have kept the aisle and sat apart from girlfriend, or lose the aisle seat.