It turns into lust, when you start to fantasize about having sex with them. If you simply think they look attractive, thats human.
**2514 ** St. John distinguishes three kinds of covetousness or concupiscence: lust of the flesh, lust of the eyes, and pride of life.301 In the Catholic catechetical tradition, the ninth commandment forbids carnal concupiscence; the tenth forbids coveting another’s goods. (377, 400)
Lust of the eyes, meaning fantasizing about a person, more or less coveting their body. Lust of the flesh is the actual sex.