The TOS Klingons don't have crainial ridges, and this was explained by Worf as "Something the Empire dosen't talk about" and that they had some sort of desease. Well Klingon women are srtong willed and horny so I'd gather a female ship ran by Klingon women captured humans at one point and took advantage of the men, not saying it couldn't be the other way around, but Klingon women taking advantage of men seems more likely. The expansion of the Federation into new space would allow for such a scenario at the time of TOS and could explain why no female or all female crews are present at all in the series, cause apparently Klingon women can't keep it in their pants.