Halakhah zu Wajikra 21:13

The commandment of a high priest to marry a virgin maiden: That a high priest marry a virgin maiden, as it is stated (Leviticus 21:13), "And he shall take a woman in her virginity." The proof that this is considered to be from the positive commandments is that which they, may their memory be blessed, said (Ketuvot 30a), "Rabbi Akiva would make a mamzer (one illegitimate) even from those liable from a positive commandment." And they elucidated that this is when a high priest has intercourse with a woman who is not a virgin, who is prohibited to him by a positive commandment. As it is a principle with us that a negative commandment inferred from a positive commandment is a positive commandment. And they, may their memory be blessed, also said (Horayot 11b), "He is cautioned about the widow and commanded about the virgin."