To be pedantic about the fuzziness of the "species" delineation, not every human is capable of producing viable, non-sterile offspring (breeding partner notwithstanding).
The fact that some individuals are sterile has nothing whatsoever to do with species delineation. If there were entire pairs of population groups, say the Ainu and the Xhosa, which can no longer produce viable offspring with each other due to generic distance, then we could begin a conversation about speciation of homo sapiens. But there aren't, so we don't.
