Gemara

(Rav Yosef): If there is an opening to Reshus ha'Rabim, there is no problem of seclusion.

(Rav Kahana): If men are in an inner room, and women are in an outer room, we are concerned for seclusion (lest a man enter the women's room). If it is vice-versa, we are not concerned.

A Beraisa teaches just the opposite!

Abaye: Out of doubt, we must be stringent and forbid both.

Sukah 25b - Question: Why are a Chasan and his friends exempt from Sukah? They can rejoice in the Sukah!

Answer (Abaye): We are concerned for seclusion.

Rishonim

Rif and Rosh (4:24): If there is an opening to Reshus ha'Rabim, there is no problem of seclusion.

Rambam (Hilchos Isurei Bi'ah 22:12): If one is secluded with a woman and there is an opening to Reshus ha'Rabim, we are not concerned for seclusion.

Ritva (Kidushin 81a DH): When men are in an inner room, and women are in an outer room, the Beraisa is not concerned for seclusion. Since men may go through, it is as if it is open to Reshus ha'Rabim.

R. Yerucham (Nesiv 23:1, 192:1, citing the Ramah): If one is intimate with a woman, he may not be secluded with her even if Ba'alah b'Ir and there is an opening to Reshus ha'Rabim. (In any case) a modest person should avoid what is revolting, even if Ba'alah b'Ir and there is an opening to Reshus ha'Rabim.

Question (Tosfos Shabbos 13a DH Mah): Sleeping together clothed in the same bed when there is no concern for Yichud, e.g. there is an opening to Reshus ha'Rabim, is less severe than seclusion. The Torah does not forbid the former with another's wife, but it forbids Yichud. We permit Yichud with Ishto Nidah from "Sugah ba'Shoshanim", but forbid sleeping in the same bed from a Hekesh to Eshes Re'ehu. We should forbid Yichud, and permit sleeping in the same bed!

Tosfos (Shabbos 13b DH Mitah): Palti Ben Layish (who was concerned lest his 'wife' Michal was really David's wife) put a sword between himself and her. Yichud was not a problem, for there was an opening to Reshus ha'Rabim.

Poskim

Shulchan Aruch (EH 22:9): If a house is open to Reshus ha'Rabim, there is no concern for seclusion with Ervah there.

Chelkas Mechokek (13) and Beis Shmuel (13): Even if there is an opening to Reshus ha'Rabim, seclusion is forbidden if there was Kinuy (her husband warned her not to be secluded with him) (Terumas ha'Deshen 244), or if they are intimate.

Taz (8): An opening to Reshus ha'Rabim does not permit seclusion if there was Kinuy, but it helps if they are intimate or relatives.

Hagahah in Taz: There is no source for the Chelkas Mechokek's stringency. The Gemara says that Ba'alah b'Ir does not permit if they are intimate, but an opening to Reshus ha'Rabim is more lenient. We cannot learn from Kinuy. The Torah is more stringent after Kinuy; it makes Safek like Vadai!

Birkei Yosef (6): The Rambam, Semag, Tur and Shulchan Aruch connote that an opening to Reshus ha'Rabim permits even if they are intimate. It seems that Tosfos (Shabbos 13a and 13b) agrees.

Be'er Heitev (9, citing Keneses ha'Gedolah): An opening to Reshus ha'Rabim permits during the day, but not two or three hours into the night. The first half-hour of the night is like day, for people are still walking outside.

Pischei Teshuvah (9, citing Birkei Yosef 9,10): This is reasonable. Tosfos says that Yichud was not a problem for Palti Ben Layish and Michal, for there was an opening to Reshus ha'Rabim. We must say that people were always walking by, day and night. An opening to Reshus ha'Rabim permits even if he is lewd.

Ezer mi'Kodesh: An opening to Reshus ha'Rabim is when the door is open, but not if it is closed but not locked. One senses when a door is opened, so he can distance himself then, so he does not fear. If so, we should forbid even without a door, for he can go to the corner! Perhaps then, he would not hear when someone enters. Rather, Chachamim did not distinguish. Whenever he has some fear, we permit Yichud. Also regarding a Nochri and wine, we are concerned only if the door is closed; we do not distinguish. Perhaps even at night, when people are not around, we do not distinguish! It seems that this is like a room open to a Midbar. If the place where many go does not extend to the opening, perhaps it must be visible from the house. In any case we must say that people commonly go from Reshus ha'Rabim to his house. Elsewhere, I wrote that an opening to Reshus ha'Rabim permits as long as the door is not locked, even if they are in an inner room and two doors separate from Reshus ha'Rabim. If there is a door to the Chatzer, people inside do not know when others enter the Chatzer. We say so about wine. It is Midas Chasidus that the doors be truly open. A window is like an opening to Reshus ha'Rabim, unless there are corners in the room that cannot be seen from outside.

Pischei Teshuvah (10, citing Chachmas Adam 127:7): If three people live in a Chatzer, it is a Reshus ha'Rabim for Tum'ah. One must investigate whether it is like Reshus ha'Rabim to permit Yichud in a house open to such a Chatzer.

Binas Adam (18): It seems that Yichud is forbidden. Even a Mavoy (alley) is considered secluded, and a Reshus ha'Yachid for Tum'ah. We forbid when men and women are in an inner and outer room, even if the door in between is not locked. It seems that we forbid even if the door is open.

Binyan Tziyon (138 DH v'Hinei): We are concerned lest the Chasan descend for his needs and someone will be secluded with the Kalah in a Sukah. Sukos were normally on the roof, and people do not normally pass through due to the exertion (Rashi Sukah 25b DH mi'Shum). Presumably, this is even if the door is not locked. Rashi teaches that there is no Isur Yichud when people can come at any time. They would fear to have Zenus. The Rashba says that one fears whenever the door is not locked. If people do not go there, it is as if the door is locked. We cannot learn from men and women in an inner and outer room, for there they do not usually go to the other room. The Gemara mentioned an opening to Reshus ha'Rabim, for this always permits. A room in an Aliyah is always forbidden even without a closed door, like a Sukah, unless people can enter without permission.

Rema (178:8): If there was seclusion after Kinuy, it does not matter if Ba'alah b'Ir or there was an opening to Reshus ha'Rabim (Terumas ha'Deshen 244).