Rashi on Leviticus 20:21/רש"י על ויקרא כ׳:כ״א

1נדה 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).