For me it's not an issue of "must". Techhnically, no one has to tell anyone any thing.
For me it's an issue of "love". I love the knowledge that there is nothing I can't share with my husband. I love the comfort I get from knowing there is one person one this big blue ball who I can kevetch to without fear of judgement.
I love knowing that the "old guy" will listen to me kevetch about any thing and every thing even if it bores him out of his skull.
It gives me great comfort, strength, support and joy.
Also let's keep some perspective, we're not talking about telling our spouses every little thing or detail. Does my dh care that I brought a new nail polish at target. No. But if my best friend decided to confide in me that shes cheating on her dh and doesn't want my husband to know? sorry toots, I don't generally blab to the old guy but I definitely ain't lying to him to keep your confidence.
So I really don't deal with conversations that start off with " don't tell your husband" because I don't want to even be put in a position of watching what I say to him. I don't want to even start to think "if I tell my dh, Jane is going to be mad".
Like I said, I have had my friends come to me and say "Please don't tell Sam" and I politely ask them to not confide in me then. I love 'em but there is nothing I feel the need to keep from my "old guy"