Sforno on Leviticus 18:24/ספורנו על ויקרא י״ח:כ״ד

1אל תטמאו בכל אלה, even by merely being physically close to such incestuous members of the opposite sex.

2כי בכל אלה נטמאו הגוים, for the original depravity of the nations dwelling in Canaan before you involved precisely this ritual contamination. They did not start by actually sleeping with the partners outlawed to them, but by only fondling them.