Jesus said,
Matthew 5:28
But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
Please note: This doesn’t say that if a man admires a woman’s beauty in his thoughts he has committed adultery. It also doesn’t say that if a man thinks about what it would be like to be with a given woman he’s committed adultery. Jesus doesn’t say that if a man has a fantasy about being with a given woman that he’s committed adultery. Nor does Jesus say that if a young adult who is engaged happens to fantasize about being with their wife-to-be that they’ve committed adultery.
Jesus says that if a man looks at a woman with the intent “to lust after her”… he’s committed adultery “in his heart”. I think that the issue is the intent of the heart. If you look upon a woman with a true intent to pursue her with the desire to use her, you’ve already crossed the line in your heart. Now, if you look at a woman and you’re star struck and in love. And you’re talking to her and getting to know her. And she’s in your thoughts. And you’re hoping she could be the one you’ll marry. You’ve not sinned at all.
Didn’t the lovers in the Song of Solomon fantasize about each other? Was it a sin?