Sefer HaChinuch ספר החינוך

To not reveal the nakedness of the father: That a man not reveal the nakedness of his father — meaning to say that he not lay with him, the layings of a woman, as it is stated (Leviticus 18:7), “The nakedness of your father [...] you shall not reveal.” And this warning (negative commandment) about one who lays with his father is in addition to the general warning of laying with males about all men (Sefer HaChinukh 209), as it is stated (Leviticus 18:22), “And with a male you shall not lay, the layings of a woman.” And so did they, may their memory be blessed, say in Sanhedrin 54a, “‘The nakedness of your father [...] you shall not reveal’ — your actual father.” And they asked there, “That is extracted from, ‘And with a male you shall not lay, the laying.’” And the one who answered, answered, “So as to make him liable for two [prohibitions], and it is like Rav Yehudah, as Rav Yehudah said, ‘A gentile who has intercourse with his father is liable for two.’” And there they elucidated and said, “It is likely that the word of Rav Yehudah is with an Israelite, and inadvertent and with a sacrifice; and that which he said, ‘a gentile,’ is [because] he took a euphemistic expression.” [This is] meaning to say that he did not want to mention this disgraceful matter with an Israelite, since it was possible to establish it with a gentile; as they are also commanded about sexual prohibitions, but the law is the same with an Israelite — that he is liable for two with his father, meaning that if he lay with him inadvertently, he is liable to bring two sin-offerings. The substance of this matter is revealed to all. There is no reason to be lengthy about its root, as it is fitting to distance this great ugliness from people, and to punish one who transgresses it [with] a great punishment. And therefore it made him liable for stoning.

From the laws of the commandment is that which they, may their memory be blessed, said (Yevamot 53b) that the liability is with the insertion of the corona immediately, but not before then. And the law is the same for all of the sexual prohibitions, that the liability with them is with the insertion of the corona immediately — except for one who has relations with a maidservant, [for] which the liability is with the completion of his intercourse. And we will inform the place of the rest of its details together with the rest of all the sexual prohibitions at the end of the Order, with God’s help. (See Tur, Even HaEzer 21.)

And this prohibition is practiced in every place and at all times. And one who transgresses this and lays with his father volitionally with witnesses and a warning is stoned from when he inserts the head of the corona; inadvertently, he brings two sin-offerings at the time of the [Temple] — one on account of the negative commandment of “his father,” and one on account of the negative commandment of “a male.”