This is just the same old story, and not only with regards to video games: someone says something, someone else gets offended and all up in arms about it, and it sparks a public debate on whether the content in question is "inappropriate for children" and/or should be banned, etc.
The answer, of course, is NO.
That's what the First Amendment (USA) is all about. Content should never be quashed, regardless of its potential inflammatory nature. The proper response to content you find offensive or disturbing is not to close your eyes, plug your ears, and pretend it never happened; the proper response is to deconstruct the content and think about WHY it is so objectionable to you. What core values and beliefs might you hold to elicit that emotional response? In this case, the father's overreaction could indicate many things. Perhaps he feels he's unequipped to protect and educate his daughter to weather the perpetual onslaught of modern culture. Perhaps he himself has not quite banished from his mind the societal notion that being adopted makes one inferior (it doesn't). Or perhaps he is indeed just a self-important tosser. Regardless, this incident says far more about him than about Portal 2, Valve, gaming, or even his daughter.
As a wise man once said, "When did 'sticks and stones may break my bones' stop being relevant? Isn't that what you teach children, for god's sake? That's what you teach toddlers! 'He called me an idiot!' 'Don't worry about it; he's a dick!'" As a kid, I was constantly a victim of bullies. I never really learned how to deal with it, and on reflection that made me worse off as a person. However, bullying - and offensive content in general - is never going away. It's a fact of life, a force of nature. Humans thrive on overcoming adversity, and if I'd had the choice between my dad somehow forcing the bullies to stay away from me and him teaching me how to defeat them myself, I'd have chosen the latter. Challenging adversity and overcoming it is how people and cultures learn and evolve.
To this guy, I say: you will not always be there to fight your daughter's battles; the best you can do is teach her how best to fight her own. Explain to her how the hateful things people say and do should not matter, that her focus should be on her own life and ambitions, not always on what other people think of her. Show her that being adopted is nothing to be ashamed of; she has a loving family and a good life, period. And for god's sake, don't go to the news. Americans have become far too litigious. You're a parent, so BE one. Your daughter needs a dad, not a lawyer.