You’re right. Not because they’re sinning, though. The kids just feel grossed out seeing their mother or father making out with another person (or even with each other).
Oh yes, it would be because they can’t brook their elders warning them not to live in “sin” — in a foolish relationship. Nobody ever invented a pill or a condom for the heart. Would you refuse to listen to a doctor’s advice to moderate or stop drinking because the doctor himself drank too much?