Leviticus 20:21

נדה הוא means, this cohabitation ought to be kept at a distance (avoided) and is loathsome. Our Rabbis explained that these words prohibit the slightest approach to her [I.e. one is liable for superficial contact, as is defined in the commentary on verse 18] just as though she were a נדה in the case of whom such intimacy is expressly stated as forbidden, as it says, (v. 18) “he hath uncovered her fountain” (cf. Yevamot 54a).

וְאִ֗ישׁ אֲשֶׁ֥ר יִקַּ֛ח אֶת־אֵ֥שֶׁת אָחִ֖יו נִדָּ֣ה הִ֑וא עֶרְוַ֥ת אָחִ֛יו גִּלָּ֖ה עֲרִירִ֥ים יִהְיֽוּ׃

And if a man shall take his brother’s wife, it is an unclean thing: he has uncovered his brother’s nakedness; they shall be childless.