It's going to be really funny if it turns out the author intentionally misled us and the father was right on his guess (spent too much time in the sauna.) It would be hilarious if it was just those Marika and Dariel being kept in the sauna late (because Marika is immune) and she put him to bed and just fell asleep next to him. Mom walks in on them and sees that and thinks they did it, when nothing happened. Marika never had sex ed, but all she's been taught is "if you sleep with a man, you'll make a baby", so now she thinks she's pregnant with nothing happening, and Dariel is just too pure. He thinks he saw her, so he must marry her.