This.
It's one line in a movie.
True. But it is one line that could be like a knife to the heart to a kid who was adopted and just wanted to see a fun movie. Consideiring how many non-adults will see this movie and how many of them will have been adopted, the line has the potential to hurt many children. And sadly, it will once again, give ammo to those who want to make fun of kids who have been adopted. Why not something like, "Loki was not born Asgard," to make the same point without the old cliche of, "He was adopted."? I fear the answer is because (1) resorting to adoption as a punchline is easy and/or (2) some people fail to realize their quietly held prejudices.
Yes, I saw Thor. I was looking forward to The Avengers. This has taken a lot of the fun out of seeing it and I am relieved we are not taking DD.