It is forbidden for a Chasan [or Kallah] to go outside alone in the marketplace [or any area, both by day or night during Sheva Brachos]. The Chasan and Kallah are to only go out with an escort. Due to this, the custom became amongst many Chassanim not to go to Shul throughout the Sheva Brachos, as there is no one found to escort them to and from Shul. [Practically, if the Chasan does not have a Shomer to escort him he is not to go to Shul.]
May the Chasan and Kallah go out alone together?
May a Chasan or Kallah be alone in their house?
May the Chasan go out alone to perform a Bris or be a Sandek?
May a child be a Shomer?
May a gentile be a Shomer?
May two Chassanim and two Kallah go out together?
 Rama 64/1; M”A 239/7; Brachos 54b “A Chasan and Kallah need Shemira”; Ran Kesubos; Pirkei Derebbe Eliezer 12; Sefer Haminhagim p. 177 [English]
 Brachos ibid; Aruch Hashulchan 64/3; Yifei Lalev 4/64-5; Sefer Haminhagim p. 177 [English]
 Aruch Hashulchan ibid; See Beis Shmuel 64/2 in name of Perisha 64/2 that this includes a Shul; Seemingly the avoidance of a marketplace fits the reason of the Chasan being a King, as Mazikin are not found there, whiule other areas where there are not people found, fits the reason of Mazikin.
 Aruch Hashulchan ibid; Seemingly the Mazikin is the reason behind not going alone at night and being like a king is the reason not to go alone by day
 The reason: As a Chasan is similar to a king who needs an escort in public. [Chelkas Michokek 64/2; Ran ibid; Pirkei Derebbe Eliezer 16; Aruch Hashuclahn ibid] Alternatively, as a Chasan and Kallah need protection from Mazikin and hence cannot go out alone. [Brachos ibid] As the Satan persecutes during times of danger and happiness. [Talmidei Rabbeinu Yona Brachos ibid] Hashem made ten Chupos for Adam Harishon in Gan Eden and the angels danced and cheered by the wedding. The angels guarded Adam and Chavah, as it says in Tehillim Ki Malachav Yitzaveh Lach Lishmarcha Bechol Derachecha, and Derachecha refers to Chasanim. [Pirkei Derebbe Eliezer ibid] There was once a Chasan who walked outside alone and he was killed, being snatched by the Malacha Hamaves. [Eirusin Unissuin of Rishonim p. 127, brought in Nitei Gavriel 56 footnote 1]
 Beis Shmuel 64/2 in name of Perisha 64/2; See Perisha ibid that some were accustomed not to leave to Shul even with an escort, although this is a Minhag Shtus; See Nitei Gavriel 56 footnote 9 and 56/8 footnote 13 that the Chasan and Kallah should only go out even with an escort when neccessary
 Beis Shmuel 64/2 in name of Perisha 64/2; Machzor Vitri 477
Custom of Sefaradim: The Seafradi Chasanim walk to Shul alone during the day without a Shomer. [Nitei Gavriel 56/1]
 So is clearly implied from Perisha and Beis Shmuel ibid, however see Halichos Shlomo 5/7 in name of Rav Sz”a that if he is found in an area that many people are in the streets he may walk to shul alone.
 Nitei Gavriel 56/6
 Sefer Haminhagim p. 177 [English] Based on a directive of the Rebbe Rayatz that a king must go out with an entourage.
 Aruch Hashulchan 64/3; Hagahos Yaavetz Brachos 54b
 Nitei Gavriel 52/7 in name of Migdol Oz
 Chelkas Mechokek 64/2
 The reason: Her Mechila does not help as he is similar to a king who needs escorts. [Chelkas Mechokek ibid] Alternatively, it is forbidden due to Mazikin. [Brachos ibid]
 Mishnas Yehoshua 25/14
 Mishnas Yehoshua 25/14
 The reason: As the reason behind this Shemira is not only due to Mazikin but also due to being similar to a king, and hence they need a Shimer who is not a king to escort them