Yup, that one. What you said is essentially all we know about it and it bugs me a lot lol. We can speculate a lore reason (it signifies pregnancy for a smyrna but only their respective capri can see the mark) but I suspect there isn't one since there's too many plot holes (why the mark shows up only during sex, why only theo has it, since if it were a common occurrence theo would've probably studied it, why there even is a mark since pregnancies generally don't create marks on your body, etc)
I thought about that too. There's no evidence, but it's the only hypothesis I could come up with. What bugs me really is that there's no clear reason for the mark to even exist and it's also never addressed, Seth points it out and moves on like it's completely normal for pregnant people to get markings during sex lmaoo maybe the author planned to include it in a side story but it ended up never being made idk

The never fulfilled promise of side stories... I feel like this one really needed them.
Also it still bugs me we never got an explanation for that damn marking. Like it seems the author added it just to look sexy but had no idea why or how it actually got there so they just avoided the subject