When does sexual attraction become lust?

According to Catholic theologians, when does sexual attraction become lust? By "sexual attraction", I mean the pleasure those of the opposite sex feel in one another's presence. By "lust", I mean "seeking venereal pleasure (*delectatione venerea*) not in accordance with right reason" (St. Thomas Aquinas, *Summa Theologiæ* II-II q. 154 a. 1 co.).