Sforno on Leviticus 20:8/ספורנו על ויקרא כ׳:ח׳

1ושמרתם את חקותי ועשיתם אותם, the only way you can ensure that this degree of your sanctity continues throughout the generations is by ensuring your meticulous observance of the עריות legislation. If you were to fail to do this, your children, or some of them being born from a sinful, spiritually contaminated union, would already have these strikes against them from birth, making it so much harder for them to attain their G’d-set goal to achieve the level of sanctity expected of them. Compare David’s reference to the subject in Psalms 51,7 where he considers the fact that one’s mother may have not have had the purest thoughts when cohabiting with her husband as already resulting in her child being born in sin, בעון חוללתי.

2אני ה' מקדשכם, by forbidding you incestuous relationships I G’d have actively helped you in your attaining this sanctity, holiness.