You shall be holy (19:2)
Sanctify yourself also regarding that which is permissible to you.
(Talmud, Yevamot 20a)
The meaning of this is that since the Torah has warned against forbidden sexual relations and forbidden foods, while permitting relations with one's wife and eating meat and wine, the lustful person can find a place to wallow in fornication with his wife or wives and be of "the guzzlers of wine and the gluttons of meat", and converse at will of all licentious things (since no prohibition against this is specified in the Torah). He can be a hedonist with the Torah's permission. Therefore, after enumerating the things which it forbids entirely, the Torah says: "Be holy." Constrain yourself also in that which is permitted.