In order to prove to Yuri that they are actually a married couple, Loid and Yor are supposed to kiss. Loid is willing to get this done immediately for the sake of Operation Strix, but Yor can't do this sober, so she ends up downing a bunch of wine and gets drunk.  Yuri sees his sister this way, also gets drunk, and is unable to make rational decisions anymore...
I see. He used a cipher because
 he was embarrassed.
.فهمت. استعمل شيفرة لأنّه يشعر بالإحراج
Verstehe.
Vor Scham hat er den Brief verschlüsselt.
Comprendo. Le da vergüenza
y por eso lo manda cifrado.
Comprendo. Le da vergüenza
y por eso lo manda cifrado.
Je vois.
Capisco. Ha scritto un messaggio in codice 
perché si vergognava.
Entendi. Ele usou a cifra por vergonha.
Вот в чём дело…
он со стыда решил всё зашифровать.