1- First of all, Islam promotes modesty and wants our honor and dignity to be protected. Islam considers it a violation of a woman’s honor for a man to look at her with lust. She is being objectified and this is not appropriate.
2- Lustful looks can lead to sin. The man may try to pursue his lust and start a relationship with her, and it could be a haram relationship and they would end up committing adultery or fornication.