My husband doesn't know my phone passcode, but that's only because he refuses to remember it. There are times he'll need/want my phone for something (usually when his is dead) and I just tell him what it is. He doesn't have a lock on his phone, but if he did, he would tell me. He needs my help too often to make it work for me not to know how to get into it. LOL.
But he trusts me and doesn't really care to take the time to read through it. 99% of the time, as I'm texting someone, I'm narrating the conversation to him anyway, so I don't have to repeat it later. And, I trust him, and he so rarely texts anyone besides me and his brother that it's not a big deal.
However, we have friends who this is a big deal for, who we snark about because of it.