I don't think there are so many restrictions on dating because they are not really public figures. Their real names are secret, they don't make appearances on TV, if they had a boyfriend nobody would know.
The other reason for the no boyfriend rule for pop stars and the like is because the management company wants them to concentrate 100% on their careers. No distractions. Not an issue with gravure, it's not a full time job and doesn't require much practice. The main thing is pressure to keep weight down and stress of not knowing how their bodies will develop as they get older, like one day they might just find they aren't able to get gravure work any more.