Okay, I'm gonna be a bit of a skeptic on this trope because, honestly, the setup is wild. The 'secret life' is almost always something massively disruptive—a mafia don, a spy, a secret royal, a supernatural being. If your spouse is hiding that for years, the marriage is built on a fundamental lie. That's not a minor detail like forgetting to mention you hate cilantro.
So how it affects the marriage? Initially, it's a crisis of trust so deep it should be unrecoverable. But in these books, the effect is usually to inject drama that forces a renegotiation of power. The 'ordinary' partner suddenly has leverage—they know the dangerous secret. The billionaire's control slips. The fun for the reader is watching that power dynamic flip, the cold, controlled billionaire becoming emotionally vulnerable because his secret (and therefore his world) is now in her hands. The marriage becomes a thrilling, high-stakes partnership instead of a transactional one. It's pure fantasy catharsis, really.