"They have fallen into a rut because they're always above board with each other."
"They have fallen into a rut because they're always above board with each other."
"they" 「彼ら」
"have fallen into a rut" 「マンネリ化した」
"because ~" 「〜為」
"they're" = "they are"
"they're always above board" 「彼らはいつも隠し事しない仲」
"with each other" 「お互いに」