John Gill Commentary Deuteronomy 27:20

"Cursed be he that lieth with his father`s wife, because he hath uncovered his father`s skirt. And all the people shall say, Amen." — Deuteronomy 27:20 (ASV)

Cursed [be] he that lies with his father's wife
As Reuben did, and which is forbidden (Leviticus 18:8); because he uncovers his father's skirt;
see (Deuteronomy 22:30); and all the people shall say Amen;
the tribe of Reuben said this as well as the rest.