It’s like you’re asking “Should you never keep secrets from your SO?” and “Are secrets ALWAYS dangerous for a marriage?”
Two extremes situations, two absolutes. So many things to consider, such as the situation…and whether or not the secret will directly impact your spouse and the marriage.
You could say that SOMETIMES it may be the right thing to divulge a secret to your spouse…as not doing so may actually hurt the marriage (especially if it were to come out later)…and SOMETIMES keeping the secret to yourself is best because it has the potential to destroy the marriage. You know that, WF. Few absolutes in the real world. Pick and choose your battles.
In my opinion, let’s say that you had past crushes and relationships…and you’ve moved on completely from them…and you KNOW that they won’t affect your marriage because you don’t care at all for your former flames…then what’s the need to discuss this information with your spouse? There’s really no need for it. Not revealing such information does not make you a bad spouse. In some situations (such as this), I’ve read on the Islamic websites…the the advisers say it’s best not to discuss such situations cuz they can backfire.
But issues such as some sort of disease (STD or otherwise) or a medical condition, for example…that will affect your spouse…it should be shared.