The idea of the verses 18-20 speaks about things that won't leave any "tracks" after they act: the eagle in the sky, the snake on the rocks, the ship in the water and relations between man and woman ( not a virgin one - עלמה so it won't leave tracks):
NIV Proverbs 30:
18“There are three things that are too amazing for me,
four that I do not understand:
19the way of an eagle in the sky,
the way of a snake on a rock,
the way of a ship on the high seas,
and the way of a man with a young woman.
Verse 20 provides an example of this kind of relation: a cheating wife. How women can cheat without feeling any shame, and continue with her life like nothing happen?:
NIV Proverbs 30:20 "This is the way of an adulterous woman: She eats
and wipes her mouth and says, 'I've done nothing wrong.'
The writer uses a metaphor from eating: it's like eating (forbidden food - IE: pork) and then wiping her mouth and continuing without any guilt saying to herself - "I didn't do anything bad."
And what is it that the writer of the proverbs finds so incomprehensible here? While the eagle, the snake, and the ship doesn't make any sin while they act - both a cheating wife and her fellow are sinning - and GOD sees everything ( also the one that eats non-kosher ). So how can they act like that?
Eating verbs/roots for sexual relations are often been used in the poetry part of the Bible - like Song of Songs etc..