It depends on the situation and what is best for each individual couple. In some cases, it may be necessary to keep a married hookup a secret in order to protect the privacy of those involved and ensure that any potential negative outcomes are minimized. However, other couples might feel more comfortable with being open about their extra-marital activities if it doesn't threaten the existing relationship or cause harm to either party. Ultimately, it is up to each couple to decide what works best for them.