And anyway, if it was an erotic novel and the point was getting readers aroused, I would understand your warning, as it wouldn't work for most heterosexual people. That not being the case, I don't see any need for a warning.
I must say, I didn't find it particularly noteworthy in Jemisin's books, to the point where I don't remember it at all (whereas some unfortunate sci-fi sex scenes will stay with me forever...)
What can I say, I simply don't agree. There are LOTS of books out there that don't even touch the subject of sex. I don't mind the concept; but I think that including any graphical representation of sex in a non-erotic novel is a cheap shot, same trick as it's used in low quality movies/tv series. Cheap way to catch viewers? Put a sex scene in it. Cheap way to spark controversy? Put lots of sex scenes in it. I simply don't appreciate measures like this. I think that porn is for sex scenes. For sex encounters needed for plot, I think it's best left for imagination only.
I much more likely to not recommend it for that.
Moreover, I told two gay friends about the potential of two penises being rubbed together in a highly-popular Hugo award-winning SF series. While neither has read SF OR fantasy before, they are most definitely embarking on this journey now.