I wonder if it's going to turn out like, someone deliberately sealed away women's ability to use magic at some point in the past, and the otoshi mark itself is actually just the seal manifesting when the individual woman has enough innate magical power to trigger it?
So then you get this abduction case, where women with otoshi marks get targeted and kidnapped, probably to experiment on them and siphon away their mana, or something. So all the dudes in the evil secret society can become more powerful. Curly-moustache boss man is probably going to be part of the bad guys too, but he'll have some twisty motivations about trying to protect the MC from the inside, or something, so I bet he'll end up redeemed in the end.
Meanwhile, maybe it'll turn out that the reason the MC can use magic is that she just has so much mana that she overwhelms the seal, making her a prime target for the bad guys. Seems plausible.