When I flew as the teenage guest of the elder Law Lord, who was large of body as well as of spirit, I got the middle seat and he got the aisle or window (whichever our pair happened to be) unless we could maneuver an aisle and window combination from the airline. Unfair? It seemed to me to be so at the time. However --

1. He paid for the tickets; I didn't.
2. He was much larger than me and would be much more uncomfortable in the middle seat than I would.
3. I wasn't his guest in the sense of hospitality (it wasn't like one's dinner host taking the larger steak and leaving the smaller one for the invited guest).
4. I could have stayed home instead of going to New York, Miami, London, San Francisco, or Boston.

On some occasions he was in F and I was in Y (never as a very small child, but from age 11 or so). He courteously explained to me that if I wanted to pay for first class then I could ride in first class. Fair enough.