Been thinking about this a bit more. 

I suppose it is meant to be a (dark) comedy. In that case, its comic elements are just not quite funny enough and/or a bit heavy handed. It still comes down to the lack of sympathy for the foreign party co-opted in as guests/victims. They're simply too boorishly wooden - OR - their boorish, wooden, qualities aren't well enough observed.

Otherwise it comes down to balance and context. Take the insemination scene. Thinking back nearly 50 years, I managed more back then with some weird chemicals inside me than I could dream of these days. But not with an audience groaning along. And not with my granny pushing on my arse to help. If you're going to put shit like that in a film then the audience needs to be clear that they can laugh along, if not, it's just a peculiar isolated incident.