It's funny: what you call "backtrack[ing] from his defense of [Loki]" is exactly what I call "a slam about adoption in general." There are only three possible meanings to the line: Loki is bad. He can be bad while we (the "real" or biological family) are good because he's adopted. Loki isn't "really" my brother or part of my family because he's adopted. Adoption is different and inferior to blood relations. Either way, the line itself rips on adoption and/or adoptees. Now, whether you think the line is funny or not, whether you think any offense was intended or not, those three possibilities are the ONLY real possibilities as to the meaning of the line. My seven-year-old daughter caught #3 and #1 without a hint of reaction or prompting from me. She's never been taught that adoption is in any way inferior to typical arrangements, so she didn't find it funny. It's a thoughtless line that is admittedly completely inconsistent with Thor's behavior toward Loki in both movies. It's not a deal-breaker for me with the movie, but having it in there is rude and insensitive. If it were about any other traditionally disadvantaged group (African Americans, women, gays, etc.) in this kind of context, it would have been widely and instantaneously disparaged and condemned. I'm sure that for Joss it was a throwaway line. For my daughter and for me, it's a comment on our family and our life. I would have been perfectly okay with it if Stark had said it and Thor had reacted violently, saying, "Do Not. Presume. To Mock. My Family. Ever Again." or something like that. The "adoption" line from a thoughtless ego-maniac, coupled with that kind of reaction from Thor about his real (meaning "his forever-adoptive-whatever-positive") brother would have addressed the issue well and had the characters behave more consistently.