That's a good point, and if it were just a poster that happens to have two men kissing (which statistically would be quite unlikely but possible), then I'd agree completely. You don't agree they should do that, fine, it's a free country and they probably don't care what you think anyway. But assuming that this teacher is continuously bringing up homosexuality in his German class, he's wrong. His job is to teach German, not to promote acceptance (or nonacceptance) of homosexuality. If he used the poster to illicit an anti-gay response from his class to make the point that in Germany homosexuality isn't considered aberrant behavior and therefore it should be equally tolerated in America, he's probably doing a good thing, making kids think rather than just accept what they have been taught while using the context of the culture whose language they are studying. There's not enough of that in school, and if he can teach children to not reflexively assume someone is bad just because others do by showing him others who don't, that's also a good thing. But if he really is continually bringing up the subject as alleged, he's using his classroom not to do the job for which he is nominally being paid, but rather as a platform for his own personal activism.
I usually agree with zsdersw about these issues, but I differ about this thread. I think there have been a number of smart, thoughtful points raised pro and con.
Also, on the point of creating a hostile environment, there are two sides to most issues. In order to prevent creating a hostile environment, one must either ban all speech, or choose which side will be correct. Those who believe differently will still be in a hostile environment, with the added stress of that hostile environment being state-enforced. So when you say someone cannot express an opinion because it will create a hostile environment for someone else, you're really saying "I am correct and my viewpoints should have the force of law."