If the only relevant pieces of information you had were the race of each man, and the average intelligence of each race, then of course it would be rational to estimate that the man from the 'smarter' race were the smarter of the two.

Even then, assuming the difference between the averages is one standard deviation of either race's distribution and each race's distribution is Gaussian, there is only 76% probability that the smarter guy is the one from the smarter race, which hardly counts as “must” in my book.