Gemara

(Beraisa) Suggestion: Perhaps one is Yotzei Simchah with birds and Menachos!

Rejection: "V'Somachta b'Chagecha" - Simchah must be through something (Behemos) from which Chagigah may be brought.

Rav Ashi: We learn from "V'Somachta" - birds and Menachos do not give Simchah.

Question: What does Rav Ashi learn from "b'Chagecha"?

Answer: He learns like Rav, that we may not marry women during the Mo'ed due to "V'Somachta b'Chagecha" - not in your wife.

Moed Katan 8b - (Mishnah): We do not marry women during the Mo'ed, whether they are Besulos (virgins) or not, or do Yibum, for it is Simchah for him.

(Gemara) Question: Why is 'it is Simchah for him' a reason to forbid marriage?

Answer #1 (Rav Yehudah or R. Elazar): We do not mix Simchos (the Mo'ed and marriage).

Answer #2 (Rabah bar Rav Huna): One might neglect Simchah of the Regel and engage (solely) in Simchah of his wife.

Abaye: This is like Rav, who expounds "V'Somachta b'Chagecha" - not in your wife.

Answer #3 (Ula): One will do excessive exertion on the Mo'ed for the wedding.

Answer #4 (R. Yitzchak Nafcha): People will (delay marriage until the Mo'ed, in order to fulfill Seudas Yom Tov through the wedding feast, and) defer Peru u'Rvu

Question: What is the source that we do not mix Simchos?

Answer #1: On Erev Yom Kipur, Shlomo made a celebration "Shiv'as Yomim v'Shiv'as Yomim". The first week celebrated finishing the Beis ha'Mikdash; the second week was Sukos. If we may mix Simchos, he would have made one celebration during Sukos!

Rejection: Perhaps one may not delay in order to mix Simchos, but one may mix Simchos if the proper time is during the Mo'ed!

Answer #2: It says "Shiv'as Yomim v'Shiv'as Yomim Arba'ah Asar Yom" - this is extra to teach that we may not mix Simchos.

18b - Mishnah: On Chol ha'Mo'ed we may write Kidushei Nashim.

Shmuel: One may Mekadesh a woman on Chol ha'Mo'ed.

Support: The Mishnah permits writing Kidushei Nashim, i.e. documents to Mekadesh women.

Rejection: No, it refers to documents of Pesikta, i.e. in which the parents of the Chasan and Kalah obligate themselves to give certain amounts to their children.

Support (Mishnah): We do not marry women during the Mo'ed.

Inference: One may Mekadesh!

Rejection: No, the Mishnah teaches a bigger Chidush, that even Nisu'in is forbidden, even though it is a Mitzvah, and all the more so Kidushin, which is not a Mitzvah.

Support (Tana d'Vei Shmuel - Beraisa): We may Mekadesh, but we may not do Nisu'in, Seudas Eirusin (Kidushin) or Yibum, for it is Simchah for him.

Yevamos 43b - (Beraisa): From the first of Av until Tish'ah b'Av we diminish Sechorah, commerce, building, planting, Kidushin, and Nisu'in.

(Rav Ashi): Perhaps the Beraisa forbids only Seudas Eirusin, but Kidushin itself is permitted!

Question: If so, also regarding Nisu'in we must say that only the meal is forbidden, but Nisu'in itself is permitted (but this is not true)!

Answer: That is not comparable. Nisu'in is Simchah even without a meal, but Kidushin is not!

Rishonim

The Rif and Rosh (Moed Katan 1:16) bring the Mishnah and all four reasons why we do not marry women during the Mo'ed.

Nimukei Yosef (DH Masnisin and DH Gemara): If one mixes Simchos, he is not properly happy with either.

Rambam (Hilchos Yom Tov 7:16): We do not marry women during the Mo'ed, lest Simchas ha'Chag be forgotten amidst Simchas Nisu'in. We may Mekadesh women during the Mo'ed, as long as he does not make Seudas Eirusin or Seudas Nisu'in, lest he mix another Simchah with Simchas Mo'ed.

Magid Mishnah and Beis Yosef (OC 546 DH Ein): The Rambam rules like Rabah's reason, lest Simchas ha'Chag be forgotten amidst Simchas Nisu'in, because Rav agrees with it.

Question (Lechem Mishneh): First the Rambam gives Rabah's reason, but later he says the problem is mixing Simchos! Perhaps he means that one will mix Simchos, and Simchas ha'Chag will be forgotten. However, in Hilchos Ishus he says that the problem is mixing Simchos! He learns from "Malei Shevu'a Zos...", like the Yerushalmi. 'Rashi' on the Rif says that the Halachah varies according to the reasons. The concern for mixing Simchos applies to all Simchos; we learn from Chanukas ha'Mikdash. Rabah's reason was expounded, and it applies only to marriage. Surely this is correct. If not, why would the Gemara seek a source that we do not mix Simchos? We already gave Rabah's source! How can the Rambam give different reasons if the Halachah varies accordingly?

Answer (Lechem Mishneh): Not mixing Simchos adds to Rabah's reason. Here the Rambam relies on what he wrote in Hilchos Ishus to forbid other Simchos as well. Alternatively, here also he forbids mixing any Simchah. At first he gives the reason to which all agree. Then he forbids Seudos accompanying small permitted Simchos, such as Kidushin or Nisu'in with Gerushaso, which is only according to the opinion that we do not mix Simchos. This is not a good answer; further investigation is needed.

Rambam (14): One may Mekadesh a woman on any day. We do not marry women during the Mo'ed because we do not mix Simchos - "Malei Shevu'a Zos v'Nitnah Lecha Gam Es Zos".

Tosfos (8b DH b'Chagecha): Perhaps the Tana of the Beraisa also expounds "b'Chagecha" like Rav Ashi to forbid marrying during the Mo'ed. Alternatively, he learns from one of the other reasons given.

Tosfos (ibid.): The Drashah ("b'Chagecha" - not in your wife) is not a mere Asmachta. We answer that Rav Ashi, who does not need "b'Chagecha" to teach about Korbanos, uses it to forbid marrying during the Mo'ed.

Poskim

Shulchan Aruch (OC 546:1): We do not marry women during the Mo'ed. One may Mekadesh a woman, as long as he does not make a Seudas Eirusin in her house at the time of Kidushin. He may have dancing and a Seudah with his friends elsewhere, or in her house at another time.

Source: Beis Yosef citing Rivash (260).

Taz (2): We are lenient about Kidushin lest another merit the woman through Divine mercy. All the more so, Tana'im are permitted. It is not clear whether or not Seudas Tana'im is in place of Seudas Eirusin, and hence forbidden. It seems that it is forbidden. We permit Seudos of Bris Milah and Pidyon ha'Ben only because the pain of the baby after the Bris dampers the Simchah and their times are fixed.

Kaf ha'Chayim (4): Some Acharonim permit Seudas Tana'im because nowadays the real Kidushin is done at the Chupah. Nevertheless, many are stringent like the Taz. Surely one need not be stringent about delicacies if it is not a proper Seudah.

Mishnah Berurah (4): It is forbidden (in her house) only if the Kidushin is during the meal or immediately afterwards. Some forbid the entire day. Certainly it is permitted on another day.

Taz (3): Seudas Eirusin is forbidden, so all the more so the first Seudah after Nisu'in (that was on Erev Yom Tov) is forbidden.