.......I dunno why, but Saki's behavior screaming 'sus' all over it.
Though this is a serialized manga, so probably won't go the route of that one (iykyk)
My suspicion is that she's after exploiting him emotionally and not sexually. When they first met she was ready to undress him, but after she figured out he was a minor, her attitude changed. My guess is that she figured out that was she was after, was closeness and intimacy you'd get from a child more than sexual urges. She's probably lonely and needs someone to cuddle and heal her heart or something like that.
Probably need to see how it goes, but from what I've read about sugar mommies, it's usually more about being wanted, seeking warmth, and closeness. In opposite to sugar daddies who are mostly after sex.