Many people in the States find the term climate change denier offensive because of its association (deliberate, they think) with Holocaust deniers. Because I know this I started this thread. Personally, I am not offended, but I don't think it's a useful term because I don't think anyone denies that climate change happens. The argument is about what causes it.
However, that is by the way. "Nigger" (for instance) used to be a term used by 'the mainstream'. It took a long time and a lot of explaining by people who found it offensive, or by others who understood their case and supported them, before "nigger" was understood by virtually everybody to be offensive. My personal view is that the term "climate change denier" comes into the same category, not because it offends me (I just think it's an easy to use but rather silly term), but because I happen to know that other people are justifiably offended by it. It denigrates their intelligent and scientific approach to a problem.