The nakedness of your father… (Leviticus 18:7)
We now begin a long series of mitzvos outlining the forbidden sexual relationships, which are called the “arayos.” The first in this list is a prohibition against a man having incest with a father. This prohibition is separate from the general injunction against homosexuality, which we will come to in Mitzvah #209.
One might ask why we don’t assume that this verse prohibits a woman from having sex with her father. Each of these mitzvos represents a reciprocal relationship. Father-daughter sex is prohibited in Mitzvah #195. This mitzvah prohibits father-son sex. (You will notice there is no separate prohibition saying “your son…” The prohibition on a father has that covered.)
Since homosexual relations are prohibited in any event, what’s the practical application of this mitzvah? It increases the severity of the act by making incestuous relations with a father a violation of two mitzvos. If committed inadvertently, one would have to bring two separate sin offerings. (I shudder to think of the spiritual ramifications if one violated this prohibition intentionally!)
The reason for this mitzvah? Certain things are universally acknowledged as “just wrong” and incest is high on that list. In the previous mitzvah, we spoke of sexual restraint. Incest shows a tremendous amount of poor judgment in addition to a lack of sexual restraint on an epic scale.
In all of the cases of the arayos, a person would be liable as soon as the male organ is inserted. At that time, any willing participants in most such acts would be subject to capital punishment (assuming that there was a valid warning and two eligible witnesses, as per standard Sanhedrin procedures).
This mitzvah applies in all times and places. In the Talmud, it is discussed in tractate Sanhedrin, page 54a. It is codified in the Mishneh Torah in the first chapter of Hilchos Issurei Bi’ah. It is #351 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #111 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.