Yes, BBBBB is rare. But BBBBG is just as rare (at least statistically. In practical terms, I suspect that having four boys in a row would be too exhausting and irritating, so they'd have given up by then).
Actually, it isn't. More boys are born than girls. (However, boys are somewhat more likely to die young.) Also, the odds for a given mating pair are not the same as the odds for the population as a whole.
And then there's post-conception sex-selection....
