Sforno on Leviticus 20:7/ספורנו על ויקרא כ׳:ז׳

1והתקדשתם, by staying clear of all incestuous relationships.

2והייתם קדושים, so that your seed will merit that the Divine Presence will dwell among the likes of him. Our sages are on record that “the Divine presence will rest only on those Jewish families that are genealogically pure.” (Kidushin 70)

3כי אני ה' אלוקיכם, the One Who said to Avraham “to be your G’d and that of your seed after you.” (Genesis 17,7) What was meant by the word לזרעך was the seed which is genealogically yours in an uncontaminated state. (as explained by Bamidbar Rabbah 12,4)