I think the OP did the right thing - you involved the FA, and it was the girl's choice not to take up the seat -I wouldn't have felt bad about the outcome either, and i certainly wouldn't have given too hoots about the attitude fro the FA.You will never see either of those people again.

On BA you can buy a "comfort seat" for quite a small amount, but you'd still have to defend it on board should someone take a shine to it because it is close to the front or an exit row. It's a bit like going to a concert or event and someone has dropped out. You have a spare seat, but its not for someone from the back row to suddenly jump into it 'cause you're in row 1.

I understand my examples are different because money has changed hands - but the principles are still the same. People shouldn't assume that just because a seat is empty they can have it. And you shouldn't feel bad for defending it.

PS: shame you can't reserve exit rows when you book really isn't it ;-)