There are certain courtesies that need to be respected even among boyfriends and girlfriends.
Even boyfriends and girlfriends need to be polite to one another to a certain degree.
この場合、次のように言えますよ。
ーThere are certain courtesies that need to be respected even among boyfriends and girlfriends.
「彼氏彼女の間でも、尊重されるべきある程度の礼儀がある」
courtesy で「礼儀・礼儀正しさ」
ーEven boyfriends and girlfriends need to be polite to one another to a certain degree.
「彼氏と彼女も互いにある程度礼儀正しくする必要がある」
polite で「礼儀正しい・丁寧な」
ご参考まで!