I don't think it's completely insane, but I think the parent comment overestimates the proportion of marriages that have that kind of trust. If people didn't get married without 100% trust then most people wouldn't be married, and evidence is that people prefer imperfect relationships to loneliness.
It's also useful to have a norm against doing these things even if you have 100% trust, because it's probably the case that people are too readily fooled (by themselves or a manipulative partner) into thinking they have 100 % trust when they actually don't. So having a norm against sharing everything protects the people who need it, even if you're not among those people.
It's also useful to have a norm against doing these things even if you have 100% trust, because it's probably the case that people are too readily fooled (by themselves or a manipulative partner) into thinking they have 100 % trust when they actually don't. So having a norm against sharing everything protects the people who need it, even if you're not among those people.