Well, normal in the developmental sense, I suppose, because it stems from something widespread among different species and VERY longstanding- so it's obviously not the result of "abnormal" socialization or abnormal physical development (because the development is according to some innate tendancy, I believe, so it's a normal development in that it doesn't deviate from the individual organism's tendancy).
__As for popular usage, tweak that dictionary; it needs it. "Normal" in popular usage is used interchangeably with "OK/acceptable," don't you think? Abnormal implies "bad/mistake." I mean, the way these words are used, normal is a better fit for describing homosexuality.