Sifrei Mitzvot untuk Ulangan 23:18

A woman is not considered to be married except by way of betrothal in which the kosher betrothal was done appropriately. However, if he were to lie with her by way of harlotry, without the name of betrothal, it is nothing (towards her status as being a married woman). Even if he lies with her with the intent of marriage, mutually agreed between him and her, she is not considered as his wife and even if she dedicated herself only for him, rather the opposite is true and he must be forced (by Beis Din) to send her away from his home. Rem"a: For certainly she would be considered an embarrassment for immersion in a mikveh and he will lie with her in ritual impurity (niddah); however, if she dedicates herself exclusively for him as his wife and she immerses for him, there are those who would say that this is allowed and she would be a Pilegesh as described in the Torah and there are those who say that this is forbidden and they should both get whiplashes from the Torah as they have transgressed the precept "don't be a harlot." A Kuti who marries a Kutis and even a Mumar who marries a Mumerres in their (non-Torah) customs and they convert (or want to follow Halacha) afterwards, we are not at all concerned that they had an appropriate betrothal and he is permitted to send her out her without a Get, even though he remained together with her many years, this relationship is not considered anything but simple harlotry (Rivash 6 and Terumat HaDeshen, Part I 209), see Halachah 149:6.

Laws of Marriage

Do not profane your daughter to make her a harlot: It is taught in the Sifra in Parashat Kedoshim [Chapter 7, and it is brought in Sanhedrin 76b]: this refers to one who hands over his daughter not for the purpose of marriage, and likewise, a woman who gives herself over not for the purpose of marriage, meaning she designates herself for prostitution to everyone. And it is taught in the latter part of the Baraita: One might think it applies to a single act, meaning if he handed over his daughter to one person to be a concubine, and likewise she, who was seduced by a seducer, violated this prohibition; therefore, it is stated, "lest the land become lewd," not a single act. This was expounded by our teacher Moses son of Maimon [in Mishneh Torah, Virgin Maiden 2:17], that which is written in the Torah, "Do not profane your daughter to make her a harlot," so that the father should not say, "Since the Torah only obligates the seducer and rapist to give money to the father, I will rent out my virgin daughter to whomever I want, or I will let this one come upon her for free, since a person can waive and give his money to whomever he wants." Therefore, it is said, "Do not profane your daughter to make her a harlot," because the Torah's obligation of money for the rapist and seducer and not lashes is only when the matter happened incidentally, without the father's knowledge, and she did not prepare herself for it, as this does not happen constantly and is not common. But one who permits his virgin daughter to be ready for whomever wants to come upon her causes the land to be filled with lewdness, and it results in the father marrying his daughter and the brother marrying his sister, for if she becomes pregnant and gives birth, no one will know whose child it is. One who prepares his daughter for this purpose makes her a harlot, and both the one who cohabits with her and she herself receive lashes for "there shall be no harlot." However, he is not fined, as the Torah only fines for rape and seduction. But this one who prepared herself for this, whether with her consent or her father's consent, is a harlot. The prohibition of harlotry applies equally to both a virgin and a non-virgin. Therefore, the Sages said [Ketubot 36b] that one about whom there was a bad report during her youth has no fine, as we wrote in Positive Commandment 54, for she is presumed to have abandoned herself to this willingly.