[potsie]

Or maybe it's the narrator trying to brush off the incident.

The narrator calls it "boring" and then a line later calls it "horrific". Rarely would one describe any horrific incident as being boring. Unless, as you suggest, it is a tactic to minimize the incident. The narrator then quickly realizes that it was ridiculous to label it as boring and in the second line he concedes the depraved nature of the act. It would work better if Paul's back-up vocals served to represent a second questioning individual in the conversation, where "It's boring (boring)" becomes "It's boring (boring?)" Probably hard to sing back-up with a questioning tone!

Or, as Moosehead suggests, "It's boring" describes the conditions that led to the incest. In which case "It was boring..." might have worked better.