Oh, come on now. I think that's a bit of an overreaction. My sister is adopted, so I was raised in a family that was sensitive to the topic, as well as to how people will react without thinking. I know the thoughtless things people can say. The funny thing is, my sister resembled my parents more than I did, so when "one of them is adopted" came out they often thought it was me!
Still, this was a quick joke, it never implied that Loki was a bad guy because he was adopted. In fact, despite the fact that he is a bad guy, his adopted brother still loves him and fights to save him every step of the way. In my opinion, Marvel owes no apologies, nor do the actors or anyone associated with the picture.
Nobody promises life won't ruffle a feather here or there. There is a chance that this light, offhand remark caused someone a pinch of pain for someone who is sensitive to being adopted. Such is life. I have never viewed adoption as a handicap of any sort, however. I believe those who say it means the children weren't wanted are not seeing the other side of the coin, where that child was desperately wanted and loved. Everyone is going to see this differently, filtered through the lens of their own experiences. Regardless, this was a comment made without malice, and in context truly was a funny comeback.
Jamie Berke, get over yourself.