The 355th prohibition is that we are forbidden from having relations with a woman without [giving her] a Kesubah and acquiring her (kiddushin).
The source of this commandment is G‑d's statement (exalted be He), "There may not be any prostitutes among Jewish girls."
This same commandment is repeated, but using a different expression, in G‑d's statement (exalted be He), "Do not defile your daughter with premarital relations." The Sifra says, " 'Do not defile your daughter' — this command is directed towards a man who hands over his unmarried daughter for sexual relations without marriage, as well as a girl who herself has sexual relations without marriage."
Now listen as I explain why the prohibition is repeated with this wording, and what the repetition adds. G‑d (exalted be He) has already instructed us in the Torah that a man who has relations with a virgin incurs none of the punishments, regardless of whether he seduced or raped her. Rather, he must pay a monetary fine and marry the girl that he harmed, as explained in the Torah.
Accordingly, a person might think that since the offender is only required to pay a fine, therefore this is looked upon as a purely financial case. Therefore, just as a person, if he wishes, is allowed to give away his money to another person, or to forgive a debt, so too, [he might think,] he may take his unmarried daughter and give her to a man to have relations with her. This would be like forgiving a debt due to him, since the 50 silver [shekels which the seducer or rapist must pay] go to the father. Alternatively, a person might think that [since this is purely a financial matter,] he may give his daughter on condition that the man pays a certain amount of money.
Therefore, the Torah prohibited this and said, "Do not defile your daughter with premarital relations." The monetary fine only refers to a case where the seduction or rape actually occurred. But it is still completely forbidden for them to engage in sexual relations, even when they both agree.
The Torah also reveals the reason for this prohibition: ["Do not defile your daughter with premarital relations,] and you will then not make the land sexually immoral, and the land [will not] be filled with perversion." The explanation of this: seduction and rape occur very rarely, but if the Torah allowed premarital relations when both parties agree, it would occur often and become widespread throughout the world.
This is a fine and wondrous explanation of this verse, and fits all the sayings of our Sages and laws of the Torah.
This prohibition, i.e. the prohibition of [having relations with] an unmarried woman, is punishable by lashes.
The details of this mitzvah are explained in Kesubos and Kiddushin.