Negative commandment · Lo Ta’aseh #347 of 613
Not to have intimate relations with a married woman
שלא לגלות ערוות אשת איש
The verse · Vayikra 18:20
וְאֶל־אֵשֶׁת עֲמִיתְךָ לֹא־תִתֵּן שְׁכׇבְתְּךָ לְזָרַע לְטׇמְאָה־בָהּ
“Moreover thou shalt not lie carnally with thy neighbour’s wife, to defile thyself with her.”
The Rambam’s words
Not to have intimate relations with a married woman, as [Leviticus 18:20] states: "Do not lie carnally with your neighbor's wife."
שלא לגלות ערוות אשת איש, שנאמר "ואל אשת, עמיתך--לא תיתן שכובתך" (ויקרא יח,כ).
Why · Sefer HaChinuch #35
To not reveal the nakedness of a man’s wife
“It is from the roots of this commandment [that it is] so that the world be settled as G-d desired. And G-d, blessed be He, wanted that all of His world make their fruit, each and every one according to their species, and not that one species mix with another species.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה