Note for the beginning of the chapter and the likely reason Marie is so flustered by the earrings: there’s a myth that, in the 80ies, wearing a sole left earring was known as a “gay ear”, a subtle dogwhistle to signal that the person is gay and only seeking same-sex partners.
I say myth because it’s inconclusive if this is actually true or not, but there is also a chance that the claims of it being a myth comes from anti-gay sources.
I don’t want to risk perpetuating attempts at queer erasure, so I’m leaving either possibility open to interpretation. With this, you have enough info to be able to do your own research and reach your own conclusions.
Whatever the case, queer manga (and possibly queer communities) in Japan tend to pick up on western trends and myths like these and adopt them. So if you see a character in a manga really highlighting that they wear their earring only in the left ear, chances are it’s a deliberate and subtle signal.