After there has been a relationship betrayal of some kind (for example an extramarital affair, or sexual behavior not shared with the partner, or pornography viewing done in the absence of a partner), this question inevitably comes up. It's one of the most important questions we talk about in my office and yet one of the most scary for couples. Why? Because initially after the discovery of the betrayal, one or both partners want complete transparency: unlimited access to the cell phone, passwords to email and other online accounts, and heck, even ankle monitors! This is a direct result of the trauma felt by the betrayed partner and observed by the betraying partner (not to mention the guilt the betraying partner now feels): something was happening to/in their relationship without their knowledge or consent and now their world has turned upside down. What they thought was clear is now murky. Scary indeed -- to both partners. But complete transparency is impossible (believe me, I've seen and heard many clever strategies by the betraying partner to avoid so-called "complete transparency") and the betrayed partner usually end up saying something like "I don't want to be in the role of the cop", meaning they don't want to monitor their partner's activities 24-7. As their couples and sex therapist, I don't want this either. This can have profound implications down the road and it's my job to think about these things. The betrayed partner wants to trust. And if the betrayer is really committed to the relationship (and not doing the betrayal as a way out the door), they generally want to be trusted again too.
So eventually we circle around to this question -- defining what each partner thinks is private vs. secret. It can be a tough conversation because each partner is asserting themselves and there probably isn't going to be 100% agreement on everything. Nor should the partners expect there to be and they need the inner resources to handle that. More importantly, this question and conversation acknowledges that there are some things you just won't know about your partner. It's a necessary conversation. And having it helps each partner grow more into themselves and their relationship.