The trouble is that he decided to express scepticism and irritation about Bruce Bagemihl's 'Biological Exuberance', that sort of memorable study of homosexuality among animals. Specifically, after showing some of the illustrations and mocking them, he pointed out that, after all, these were drawings and therefore not evidence.
Two points - the book has quite a lot of photographs and some of them are as specific as the various drawings, notably the photo of two male walruses flipping each other off. The second is that the sort of book it is, a serious study of ethology, will often use artist's impressions of observed behaviour that was not convenient to photograph. This may not be a good thing - but it is a feature of ethology generally, not just Bagemihl.
Also, Gervais is really far too intelligent to be let get away with the assumption that, when two female marmots are humping each other, nothing much is happening. His audience ought to be better than that as well.
Just so we don't forget that much of the middle-class stand-up-going audience thinks it funny to assume lesbians don't really get to have sex at all... Dream on, sunshines.