Do you know why only females have cheek marks? Because Butterflies are a matriarchal group and pass the wand to other females meaning, the exposure is only ever on them consistently and so, eventually they developed permanent cheek marks because their family has been constantly bombarded with magic.
There's no reason to presume males can't develop them had the situation been patriarchal and there's no reason to suggest that males can't carry the gene onto female progeny.
Marco, if he is a butterfly, has the cheek marks there because, as a male, he hasn't got real permanent cheek marks. However, as a butterfly he carries the gene. Exposing himself to magic activates the dormant marks, which is not dissimilar to the exposure early butterflies would have received.>dormant carrier of a gene that is active only in females>said gene is a mutation of an otherwise bodily reaction to consistent magic exposure>dormant gene exposed to magic, as per its creation>reaction takes place as it would have in females before the mutation occurred
It makes perfect sense. There's no odd parts about it.