All forbidden sex falls under the general rubric of “Erva.” Literally nakedness, revealing, uncovering what should be covered. There are no subdivisions, like incest, everything, from adultery to homosexuality comes under this term. And yet there are a lot of other words used in the Torah to express disapproval.
In this chapter alone you have in addition to Ervah, Zima, Toevah, Tevel and later on Hesed. We actually do not know or cannot agree on exactly what the differences are between these terms. But they do all express disapproval. But why?
One answer is that the Torah wants sex to be confined to marriage and creating an ideal family for raising children. The Torah often expresses an ideal even if it knows that often humans cannot live up to it. The Torah is tolerant of those who cannot adhere to them. For example, of someone who says, “I do not want to get married” if the motives are pure. We do not punish them or discriminate.
Family life is important. Making sure there is minimal discord and conflict. Knowing how and from whom children were conceived and indeed trying to avoid the problems that having sex between people very closely related, creates. There are lots of potential difficulties.
Because sexuality is so seductive, and we are inclined to argue to permit ourselves things that otherwise we might not. That is why the Torah expresses its disapproval in such aggressive terms and such unusual language. Precisely because the repercussions of making the wrong decisions are so grave.
The Torah was not written at a time when new knew about genetic influences. But the Talmud knew about people of different physical characteristics that were not usual or normal and dealt with them very sympathetically. Nevertheless, it retained its standards as a template even while making allowance for individual tolerance, understanding and exceptionalism.