Gemara

(Rav Yehudah): If one (who is not a Chacham) wears the Talis of a Chacham (it totally covers his cloak), he does not enter Hash-m's place. It says here "v'Lo Yinveh", just like "El Neve Kodshecha."

Berachos 16a (Mishnah): A Chasan is exempt from Shema from the wedding night until Motzei Shabbos, if he did not yet have relations.

R. Gamliel said Shema on his wedding night. His Talmidim asked "you taught that a Chasan is exempt!"

R. Gamliel: I refuse to exempt myself from accepting Ol Malchus Shamayim, even for a moment!

Pesachim 54b (Mishnah): In a locale where people work on Tish'ah b'Av, we work; in a locale where people do not work, one may not work. In every place, Chachamim do not work.

R. Shimon ben Gamliel says, everyone should act like a Chacham.

55a - Inference: R. Shimon ben Gamliel is not concerned for haughtiness, and Chachamim are concerned.

Contradiction (Mishnah): If a Chasan wants to say Shma on the first night, he may;

R. Shimon ben Gamliel says, not everyone is fitting to flaunt his piety.

Answer #1 (R. Yochanan): The opinions must be switched.

Answer #2 (Rav Shisha brei d'Rav Idi): The opinions need not be switched. Chachamim hold that on Tish'ah b'Av, since only Chachamim are not working, it is haughtiness (for an Am ha'Aretz) not to work. Haughtiness does not apply to Shema, for the Chasan acts like everyone else (who say it that night);

R. Shimon ben Gamliel holds that haughtiness applies to Shema. It requires intent, so one who says it flaunts his ability to concentrate, but we know that a Chasan cannot! On Tish'ah b'Av, being idle does not show that he calls himself a Chacham. Many people have no work to do!

Rishonim

Rif (Berachos Sof Perek 2): The Halachah follows the first Tana (who allows a Chasan to be stringent).

Rambam (Hilchos Kri'as Shema 4:7): Anyone who is exempt from Kri'as Shema. He may opt to be stringent and say it, if his mind is settled. If he is exempt because he is befuddled, he may not read until his mind is settled.

Rebuttal (Ra'avad): What is the problem if he is befuddled? (If he is not Yotzei,) he is like one who reads Torah, and unlike one who casts the yoke of Heaven off himself! This is unlike Tefilah. Nowadays haughtiness does not apply, for it is known that most people read (Shema) and pray by rote. Also Chachamim of the (Yerushalmi) Talmud said 'we should be thankful to our heads, for they know to bow when we reach Modim.'

Lechem Mishneh: The Ra'avad means that everyone says Shema, including people who say it without Kavanah. In the days of the Gemara, some said it with deeper thoughts, without Kavanas Tefilah.

Rosh (Berachos 2:15): The Halachah follows R. Shimon ben Gamliel.

Poskim

Shulchan Aruch (OC 70:3): One who married a virgin is exempt from Shema for three days if he did not yet have relations, for he is distracted with a Mitzvah. This was in previous times. Nowadays, also others do not have proper intent, so also he reads.

Beis Yosef (DH v'Im): The Rosh rules like R. Shimon ben Gamliel. He rules like Rav Shisha (who does not switch the opinions), for he is Basra. The Halachah always follows R. Shimon ben Gamliel in the Mishnah. The Rif and Rambam rule like Chachamim. They hold that the Halachah follows R. Yochanan, for he is cited in the Gemara far more than Rav Shisha. If so, R. Shimon ben Gamliel holds that a Chasan may be stringent, and the Halachah follows him. The Rif says that the Halachah follows the first Tana, i.e. based on our text of the Mishnah. Tosfos rules like Rav Shisha, and like Chachamim, but says that we never have good Kavanah, so also a Chasan should say Shema. If he would not, this would connote that at all other times he has Kavanah. This would be haughtiness!

Magen Avraham (2): The Beis Yosef says that all hold that the Halachah follows R. Shimon ben Gamliel in Mishnayos. In CM (163:1), we rule unlike R. Shimon ben Gamliel! This requires investigation. The Beis Yosef (CM 231:8 DH ha'Siton) says that there the Rambam and Tur rule against R. Shimon ben Gamliel, for the first Tana's reasoning is better.

Beis Yosef (DH veha'Rambam): The Tur cites the Rambam (one may read only if his mind is settled like others). Perhaps he teaches that nowadays, just like before; one may not read if he is more befuddled and unable to concentrate than others. The Rambam permits to read even if he is exempt, as long as his mind is settled. We follow the Rif and Rambam.

Shulchan Aruch (72:2): We do not take out a Mes for burial shortly before the time to read Shema.

Taz (1): Rav Yosef was taken out shortly before the time (Berachos 19a); the Gemara said that a distinguished man is different. The Rambam brings this distinction, but the Rif and Rosh do not. The Beis Yosef says that it is because nowadays we do not have a distinguished person. I say a general rule. One may consider himself to be distinguished to be stringent, like we say about Tish'ah b'Av, without concern for haughtiness. If the stringency leads to any leniency, nowadays no one is distinguished to be lenient. Nowadays we do not fine a Litra of gold for disgracing a Chacham, and no Chacham may permit Nedarim alone.

Rema (OC 128:45): One may not use a Kohen even nowadays, for this is like Me'ilah with Hekdesh, unless he pardoned his honor.

Taz (39): A case occurred in which a Kohen served R. Tam. A Talmid asked that this is like Me'ilah, and R. Tam answered that Kohanim do not have Kedushah nowadays. Really, the Kohen was happy to serve Kodesh. This is his Kedushah! R. Tam did not want to say 'it is permitted because I am a Chacham.' In some cases, a lender may eat the Peros of Mashkanta if he deducts from the loan; a Chacham may not eat even if he deducts. Ravina ate and deducted (Bava Metzia 67b). R. Tam explained that Ravina did not want to consider himself to be a Chacham.

Shulchan Aruch (240:1): The Onah (frequency of marital relations) of a Chacham is every Shabbos night.

Magen Avraham (3): This is because on a Yom Chol, he must learn. One may not make himself a Chacham, for the Torah forbids reducing her Onah (Sefer ha'Kaneh).

Mishnah Berurah (6): If he has her permission, he will be blessed for conducting like a Chacham. Sefer ha'Kaneh was unsure if nowadays anyone is a Chacham for this law. He concludes that if he is considered a Chacham in the generation, and she married him with this intent, he is liable only on Shabbos night.

Bi'ur Halachah (DH Talmid): Me'il Tzedakah says that anyone who learns constantly, whether he teaches or learns, is obligated only on Shabbos night. Even though nowadays, regarding many laws there is no Chacham, one may make himself a Chacham for this. The Magen Avraham forbids one who does not learn to make himself a Chacham. However, Me'il Tzedakah counseled those close to him to fulfill a worker's Onah of twice a week. Also Chachmas Adam says so. The general rule is, it depends on the man's strength.

Shulchan Aruch (554:22): In a locale where people work on Tish'ah b'Av, we work; in a locale where people do not work, one may not work. In every place, Chachamim do not work. Anyone who wants to act like a Chacham in this matter may do so.

Beis Yosef (DH v'Garsinan): Hagahos Maimoniyos (5:10) says that the first Tana is concerned for haughtiness, and R. Shimon ben Gamliel is not. Tosfos says that nowadays even the first Tana agrees that anyone may act like a Chacham, for we do not work so much also on other days. Haughtiness applied only then, when everyone used to work. However, where everyone works, e.g. in Sefard, one must investigate whether one may be idle on Tish'ah b'Av. Perhaps also there, the custom is not to work (Semak). I say that everywhere, the custom is not to work.

Mishnah Berurah (43): The custom not to work is so one will not cease to mourn over the Churban. Therefore, we do not work even at night. We refrain only from Melachah that takes time to do.

Kaf ha'Chayim (100): When acting like a Chacham is haughtiness, it is forbidden. See what I said above 551:129 (that if one wants to cease being stringent, he must do Hataras Nedarim).