I mean, when your first thought after being treated that way is to, then start doing exactly that instead of taking a step back from relationships altogether until you meet someone decent, or your situation changes, I'd say that's a red flag, in my opinion at least
we don't really know how far in the past Rio had her bad experiences or how long she has sought purely physical relationships, so there's no way to say if she came to a hasty decision regarding her position on relationships.
I disagree regardless, however, as it doesn't come off to me as a purely self-gratifying or one-sided activity for her anyways, given she has a personal conversation with that guy. It indicates he is probably someone she's known for some time, so she's not being reckless or what you might perceive as "whoring herself," and there must be some kind of mutual, trust based agreement between them.
Besides, it's not so much a red flag as it is a conflict of interest for not just Seto, but anyone who would be seeking a romantic relationship with her. They want that, she doesn't.