Negative commandment · Lo Ta’aseh #353 of 613
To not marry one of all the sexual prohibitions
שלא ליקרב לעריות בדברים המביאין לידי גילוי ערווה, כגון חיבוק ונישוק ורמיזה וקפיצה
The verse · Vayikra 18:6
אִישׁ אִישׁ אֶל־כׇּל־שְׁאֵר בְּשָׂרוֹ לֹא תִקְרְבוּ לְגַלּוֹת עֶרְוָה אֲנִי ה׳
“None of you shall approach to any that is near of kin to him, to uncover her nakedness: I am the Lord.”
The Rambam’s words
Not to be intimate with a woman with whom sexual relations are forbidden in matters that might lead to intercourse - e.g., embracing, kissing, winks, or signs - as [Leviticus 18:6] states: "No person shall approach a close relative to commit a sexual offense.” Based on the oral tradition, we have learned that this prohibits intimacy that might lead to sexual relations .
שלא ליקרב לעריות בדברים המביאין לידי גילוי ערווה, כגון חיבוק ונישוק ורמיזה וקפיצה, שנאמר "אל כל שאר בשרו, לא תקרבו לגלות ערווה" (ויקרא יח,ו); מפי השמועה למדו שזו אזהרה לקריבה המביאה לידי גילוי ערווה.
Why · Sefer HaChinuch #188
To not marry one of all the sexual prohibitions
“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.”
Who & when
“And [it] is practiced in every place and at all times by males and females; as it is forbidden for them as well to have improper thoughts about men besides their husbands.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה