I don't understand why this is selfish, especially since you are taking the chance of ending up separated by a stranger. Most people assume that they can get that person to swap for either the aisle or window, so you end up no worse off than if you had just selected the seats together, and the third person gets a better seat than they expected.
I'm also not sure why the airline would care. Do they think someone would actually choose not to fly if only middle seats are available? This does not seem to be the case with US domestic flights, and as far as I know all airlines allow you to choose whatever seats you want, although the aisle and window may be priced a bit higher if there is a fee for selecting them.