the discussion changed from an explanation of the attribution bias into a series of multicultural examples in favor of moral relativity.

You should characterize such discussions as "advanced", and briefly comment on the major emotional, social, and status biases that go into such questions. When they have some understanding of their cognitive biases around questions of facts that they have no emotional investment in, then you can start talking about social and value laden biases, and maybe try some discussions where they are operative.

It's your party. They're the guests. When people are talking off topic, politely inform them of the agenda and move on.