The point about denying genocides, imho, is that (though we may be forgetful, and in some cases countries and peoples may be happy to forget), the principal case where there is an outright, organized effort to deny, or reduce, or relativize, a genocide concerns that of the European Jews. I'm referring to the movement known as "revisionism" or more properly "negationism". The overall tendency of negationism is antisemitic in that it promotes the notion of a Jewish plot to publicize, exaggerate, or (for the extremos) downright fabricate the history of the genocide. Secondly, it's a movement which favours the racist extreme right by inducing a state of confusion in people's minds as to the ultra-right's past crimes, thus disculpating the racist/fascist elements today and encouraging them to be more extremist and have the "courage" of their "convictions".

Europeans may be sensitive to this because we see the emergence of extreme-right parties and groups, of skinheads and neo-Nazis, things we thought we would never see again. And to be sensitive to how the genocide of the Jews is discussed does not imply insensitivity to other genocides and massacres, including those perpetrated by the Nazis on other groups than the Jews.

On the Congo, though the topic was not specifically genocide, here's a comment I made in a diary by Richard Drayton some time back.