They really aught not to, though, Living forever, like polyamory, is a preference which hinges strictly on a person's utility function. It's perfectly possible for a rational agent to not want to live forever, or be polyamorous.

But we're talking about probability, not possibility. It's possible for a mammal to be bipedal; but evidence for quadrupedalism is still evidence for being a mammal. Similarly, it's possible to be irrational and polyamorous; but if the rate of polyamory is greater among rationalists than among non-rationalists, then polyamory is evidence of rationality, regardless of whether it directly causally arises from any rationality-skill. The same would be true if hat-wearing were more common among rationalists than among non-rationalists. It sounds like you're criticizing a different attitude than is TheOtherDave.