I think it's really the pressures of the uneasy alliance that has gotten everyone into such a horrid mood. Normally they'd be fine, but the prospect of dining with these Klingons in a disagreeable situation put everyone in a bad mood.
Such personal situations bring out the worst in everyone and we get very judgmental. It's not hard to imagine that Kirk and company, already stressed with the volatile political climate, would not be able to resist their more base prejudices. At least they weren't commenting on it explicitly.