I read this story more than 40 years ago. Actually I read it in French, but it was almost certainly translated from english. I remember very little. It was possibly short fiction but maybe even a full novel. Humans had colonised a planet and more or less enslaved a local sentient species to do all the menial jobs. But the enslavement was not so harsh.
The only scene I remember is that one local got one day off, because he was needed for "une monte", a copulation. The point was that seven individuals were needed for that, one of each different sex. To procreate, they needed two distinct types of males, two distinct types of females, one hermaphrodite and two distinct types of "neutrals", one of which was for a "digestive" role. And I remember that these aliens were said to be "amorphous", though it is not clear what was really meant by that adjective, in that book.
I don't think the many sexes of the aliens was an important feature, it was just an incidental remark about the day off granted to the alien - the day off itself was probably a more important point. In fact I was just reminded of the "many sexes" by this recent piece of news.
It is not "Venus and the seven sexes", none of the details fit.
I have found some information. Googling a sentence in french which I thought I remembered, I ended up to an issue of a french magazine, "Galaxie" published in january 1969 (#56), containing only translations from english. The table of content mentions six short stories:
Philip José FARMER, La Quête de la vérité (Riverworld)
Piers ANTHONY, Dans les crocs du danger (In the jaws of danger)
Cordwainer SMITH, Pensez bleu, comptez bleu (Think blue, count two)
James Graham BALLARD, La Statue qui chantait (Mobile / Venus Smiles)
Terry CARR, Les Robots sont là ! (The robots are here)
Thomas Michael DISCH, Assassin & fils (Assassin & Son)
It did not look at all like a story from Riverworld. I found enough about the stories by Cordwainer Smith and J. G. Ballard to eliminate them. I don't remember any robots so it is probably not the one by Terry Carr either. But I was not able to find enough about the two remaining stories, by Piers Anthony and T. M. Disch to determine if any of those could be the one. And of course the Google search that led to this magazine may be mistaken. Still, does this help ?