On one of the I Thee Dread posts from yesterday, the linked article had a poll of 2,000 couples who said that being happy to share phone codes and media passwords was one of the keys to a happy marriage.
Jessica Coen disagreed:
Oh no. No, no, no. Why is this necessary or helpful? Sure, neither party should have anything to hide, but the idea that it plays a role in a relationship suggests insecurity or suspicion on someone's part, feelings that are decidedly not productive elements in any relationship, married or not.
I think (as I said in a comment over there) that in a healthy, non-abusive situation, being happy to share your passwords shows a certain level of trust and comfort with your partner. I've given my passwords at various times to my family and close friends so that they could help me write emails when I didn't have internet or transfer money to different bank accounts or text other friends while I was driving.
I don't get the assumption that if your loved ones have your passwords that they'll automatically snoop. If I'm happy to give you my passwords or phone code that means I trust you and assume you'll only use my password when necessary. It isn't about not having stuff to hide, it's about trusting them not to snoop.
I have a lot of passwords and phone codes from friends and family. I don't abuse the privilege by violating their trust and snooping around.
If your partner is demanding passwords, I can see that as being unhealthy and suggestive of insecurity and suspicion. But I feel like if you are happy to do it, as the poll phrased it, I think it definitely shows that you have a certain level of comfort and trust with your partner.