There's nothing selfish about it. The seats are available and open to being booked by anyone. Booking a window and aisle and hoping for the best is standard practice for my wife and me. I am not bothered in the slightest if the self-appointed hall monitors consider it ill-mannered.

And as someone who has been assigned a middle-seat only to find it was between a couple that had booked aisle and window I was more than happy to take my impromptu "upgrade."

If I and my wife book two aisles across from each other, are we violating courtesy by not sitting on the same side of the aircraft and gaming the system by sitting "together" while not taking a middle?

Of course it isn't a violation of courtesy or anything else. If my wife and I are in the main cabin we generally take aisle-across seats.

Well, I don't think so either obviously. But what's the difference between taking two desirable seats separated by an aisle and two desirable seats separated by a not so desirable seat. I don't get the idea that if two people are booking together and one doesn't book the undesirable seat it's considered by some to be discourteous or selfish.

The difference is that nobody sits in the aisle.
When you reserve two aisle seats you reserve two seats. When you reserve an aisle and a window you try to reserve three seats.

Not at all, it's a completely diffferent scenario from booking an aisle and a window as posted above as others mention.

It shouldn't need a "hall monitor" to let someone know their behaviour is boorish, but once pointed out it takes a certain kind of bad manners to continue.

Right. You're the only one(s).

My wife and I do it too, and for exactly the same reason. It has nothing to do with who might or might not sit between us, and everything to do with our individual comfort. She is more comfortable at the window, I prefer the aisle. If we were both window people, I would book us in consecutive rows, or at opposite ends of the same row.

So now we are having the same pointless conversation across two threads!

The Official Rules* are, a couple may freely book a window and aisle in the hope of having the middle free, provided that if they are unlucky and the middle seat is assigned:

(a) They ask politely if the middle seat occupant would mind switching
(b) The switch is entirely at the discretion of the middle seat occupant (ie they may choose to switch to window or aisle, or not switch at all). The requesters must accept this decision as final without protests, eyeroll, DYKWIA or similar behaviour.
(c) If the middle seat occupant declines to switch, the aisle/window occupants must under no circumstances chat, pass things, intrude on the middle space or in any way make the middle seat occupant uncomfortable.
(d) Also, the middle seat occupant may not "punish" the requesters if for some bizarre reason think they hold the high moral ground and/or the requesters are breaking some sort of sacred moral code (they aren't). No belching, grunting, spreading, snoring, getting up 15 times for the loo etc. Just behave like a normal decent person already.

(*According to me, and all sensible people)

Now, stop it, and get a life, you people.

Mrs Sopwith and I have done the aisle/window seat choice a few times. As long as airlines persist in charging for seat selection (directly or indirectly) I'm going to select what we want from among what is available at the time. And we don't offer to switch because that wasn't the point. We do tend to minimize crosstalk and passing things, but I don't see it as any different than a FA passing things back and forth across two seats.

If someone is unlucky enough to get the seat between us, that comes with the territory as far as I'm concerned. Next time pay the $ to select a seat, or book earlier if you have to.

We've also done the two adjacent aisle seat choice a few times and feel the same way. If they're available we'll take them without compunction if that's what we want.

I've put up with enough crap from other rowdy groups who heckle each other over two or three rows, stand in the aisle to chat with the person next to me or across from me with their butt in my nose, kids kicking my seatback, etc. I don't need any more crap from people who are offended when we sit quietly one seat apart.

You might sit quietly, but you sure seem to have an attitude about it.

Hell is other people!
- JP Sartre, "No Exit"