。
First item on the agenda: the matter of this morning's bed-sharing commotion.
Literal
First first [gen-の] agenda-item [topic-は] this-morning [gen-の] bed-sharing commotion [gen-の] matter [about-について].
まず最初 stacks two 'first' adverbs (まず + 最初の) for emphasis — grammatically fine and characteristic of casual opening lines. The sentence deliberately hangs unfinished on について 'regarding,' mimicking a meeting-agenda caption rather than a full finite sentence. 同衾 'sharing a bed' is a mildly archaic formal word whose comic effect here comes from juxtaposing it with the pseudo-official 議題 'agenda' framing — over-formal vocabulary deployed for laughs.