This has been bothering me, so I did some online research on the case, and it's actually pretty complex. First, for those who said they didn't see why a German class would discuss religion, I have to point out that every language teacher I know manages to work in some education about the history and culture of the country where the language being studied originated. Germany was, of course, the home of Martin Luther and the birthplace of the Protestant Reformation; I can see how a discussion on the history of Germany could have included something about religion.
However, from what I've read online, there was much more to this story than that. It happened at a public school. The comment was made to the student's friend as an aside, not as an actual part of the discussion. The teacher apparently has a history of bringing up homosexuality in his classes, and may have had some inflammatory material displayed in his classroom. As I suspected, the parents did indeed get a lawyer. And the school has reversed the suspension.
Lots to think about, in any case. Where should the line be between opinion and out-of-bounds?
Here's a link to a bunch of news articles on the case.