Gemara

Question: How could the Beraisa suggest that even a single girl is burned? It says "Liznos"!

Answer: The Tana holds like R. Eliezer, who says that if two singles have Bi'ah without intent for Kidushin, she becomes a Zonah (and is forbidden to Kohanim, i.e. this is called Zenus).

Berachos 22a (R. Chanina): Takanas Ezra was a great fence (against Zenus)!

Once, a man asked to have Bi'ah with a woman. She rebuked him 'there is no place to immerse here (you will be unable to learn afterwards)'! He desisted.

Yevamos 61b (Beraisa - R. Eliezer): A Zonah is like her name (an adulteress);

R. Akiva says, she is a woman open to have Bi'ah with anyone...

Chachamim say, it is only a convert, a freed slave, or one who had Bi'as Zenus;

R. Elazar says, if two singles had Bi'ah not to make Kidushin, she is a Zonah.

(Rav Amram): The Halachah does not follow R. Elazar.

Rishonim

Rambam (Hilchos Ishus 1:4): Before Matan Torah, a man encountered a woman in the market. If both desired, he would pay her hire and have Bi'ah with her, and go away. This is Kedeshah. The Torah forbade this - "Lo Tihyeh Kedeshah." Therefore, anyone who has Bi'ah with a woman for Zenus without Kidushin is lashed mid'Oraisa for Kedeshah.

Rebuttal (Ra'avad): Kedeshah is (a harlot) ready and Hefker for every man. If a woman designates herself for one man, there are no lashes or Lav. This is the Pilegesh of the Torah. Some forbid, and say that one is lashed for "Lo Tihyeh Kedeshah." The Isur is only if she is open for everyone. According to the Rambam, why does one who entices a virgin Na'arah pay a fine? He is lashed! Rather, since one pays for enticing, this is not a Kedeshah.

Magid Mishneh: The Sifri forbids Bi'as Penuyah (a single woman) due to the Lav of Kedeshah, like the Rambam says. Perhaps it refers to a woman ready for this, like the Ra'avad says. We often rely on the Chazakah that a man does not have Bi'as Zenus (e.g. Yevamos 107a). This must be because it is forbidden mid'Oraisa, like the Rambam says. Not everyone is careful about Isurim mid'Rabanan! The Halachah does not follow R. Elazar, who says that Bi'as Zenus makes a Penuyah a Zonah, but surely there is a Lav. If the Torah permitted it, how could she become a Zonah (according to R. Elazar)? However, we could explain like the Targum (Kedeshah is a Shifchah), and say that Bi'as Zenus is an Isur Aseh. One who wants to have Bi'ah should marry her - "Ki Yikach Ish Ishah v'Vo'alah".

NOTE: We find that a Chalal disqualifies a woman from Kehunah, even though he is permitted to her!

Rambam (Hilchos Na'arah Besulah 2:17): A man should not say 'since one who entices or rapes a Besulah only needs to pay her father, I will charge this man to have Bi'ah with my daughter', or 'I will pardon this, and let him have Bi'ah with her for free.' Therefore, it says "Al Techalel Es Bitcha Lehaznosah." One pays only money, but is not lashed, when it is happens by chance. If one leaves his daughter for whoever will have Bi'ah with her, he fills the land with Zimah. If she will become pregnant, the father will not be known. One might marry his daughter or sister! If one prepares his daughter for this, or she designates herself, this is Kedeshah. She and the Bo'el are lashed for Lo Tihyeh Kedeshah. We do not fine them, for the Torah fines only for rape or enticement. This is whether or not she is a Besulah. Chachamim said that a girl of ill repute in her youth does not receive a fine, for the Chazakah is that she willingly made herself Hefker for Zenus.

Rebuttal (Ra'avad): Kedeshah is only if she sits in a harlot's booth. The Sifri explains Kedeshah to be like "ha'Kedeshah Hi b'Einyaim Al ha'Derech."

Kesef Mishneh: The Sifri could hold that it suffices that she is ready for Zenus, and arouses men to this! In Hilchos Ishus, the Rambam connotes that there are lashes for any Bi'as Zenus, even if she is not prepared for Zenus. It is difficult, but we must say that he relied on what he wrote here.

NOTE: Also the Beis Shmuel (EH 26:2), Gra (26:8) and Maharit say that the Rambam discusses only one prepared for Zenus.

Gra (26:8): The Ramban says that Lo Tihyeh Kedeshah and Al Techalel Es Bitcha Lehaznosah apply only when she could not have Kidushin with him. Esnan Zonah (a harlot's hire) is disqualified from being a Korban only when they could not have Kidushin. Sanhedrin 21b asked that seclusion with a Penuyah is forbidden mid'Oraisa. It did not say that Bi'ah with her is forbidden mid'Oraisa! The Rambam agrees that there are no lashes if she is not Hefker to all, and all the more so if she is designated for him.

Rashi (Berachos 22a DH she'Tova): The man wanted to have Bi'ah with a Penuyah. Chachamim forbade seclusion with a Penuyah.

Rivash (425): The Rambam lashes for Kedeshah anyone who prepared herself for Zenus, even if she did not sit in a harlot's booth or make herself Hefker to all. Some say that it is an Isur Aseh. Some forbid mid'Rabanan. Some permit if she is designated for him, i.e. Pilegesh. No one permits (without designation). Even seclusion is forbidden mid'Rabanan! Chachamim did not enact Tevilah for Penuyos, lest Zenus be light in their eyes!

Poskim

Shulchan Aruch (EH 26:1): A woman is an Eshes Ish only if she had proper Kidushin. Bi'as Zenus not l'Shem Kidushin is nothing. Even if he had Bi'ah with her l'Shem Ishus alone (without witnesses), she is not his wife.

Beis Yosef (DH u'Mah): The Tur says that this is Zonah of the Torah. This is like R. Eliezer (Yevamos 61b), but the Halachah does not follow him! Rather, he does not refer to a Zonah forbidden to Kohanim. Rather, this is Zenus mentioned in the Torah, e.g. "Zinsah Tamar."

Drishah (DH u'Motzasi): Above, the Tur said that Chayavei Lavin do not make a Zonah. Here, he says that Bi'as Zenus makes Kedeshah! Perhaps above he discusses one who was not Hefker, just happened to have Bi'as Zenus. However, the Rambam connotes that (after Matan Torah) even this is Kedeshah! Some say that the second Bi'as Zenus makes her a Zonah. If so, the Tur should have specified. Rather, if she willingly has Bi'as Zenus, we assume that she will be Mezanah with others, i.e. Mufkeres. The Tur wrote 'l'Shem Zenus', as opposed to 'not l'Shem Kidushin', to hint to this.

Taz (1): Tana'im argue about what is a Zonah. R. Akiva says it is Mufkeres, i.e. she was Mezanah twice. The Halachah is not like R. Elazar, but we follow R. Akiva. The Tur says that the Rosh rules like Chachamim (Zonah is only from Chayavei Kerisos), but the Rambam says that a Lav Shavah b'Chol makes a Zonah. One who was Mezanah twice transgressed the Lav of Kedeshah. The Tur did not mention a second Bi'ah, but this is what Mufkeres means.

Chelkas Mechokek (1): The Ra'avad agrees that Bi'as Zenus is forbidden mid'Rabanan. David's Beis Din forbade even seclusion! Divrei Kabalah forbid even looking - "Bris Korati l'Einai u'Mah Esbonen Al Besulah." Terumas ha'Deshen (209) says that there is no Isur Torah of Zenus with a Penuyah.

R. Akiva Eiger: It seems that Rashi in Berachos agrees.

Rema: If he designates a woman and she immerses for him, some permit. This is the Pilegesh of the Torah. Some forbid, and lash for "Lo Tihyeh Kedeshah."

Beis Shmuel (2): Even though one is not lashed mid'Oraisa for Bi'ah with a Nochris or Shifchah (EH 16:1,3), we do not compare Arayos to each other. Each is a Gezeras ha'Kasuv.