Even 'with permission' is unethical in my opinion, because asking "can I go through your phone?" Is a loaded question, any answer but "of course" can be twisted into meaning you have something to hide. My ex girlfriend essentially forced me to give her access to my phone that way and it was a big breach of trust for me - I believe a relationship should be a partnership of two well adjusted individual people, not a complete abandonment of the self for the sake of 'love'. You should not need to sacrifice your individuality, personality, privacy, or passions for your relationship, that's not love, that's moloch.

Anyway, my take on snooping is that in some situations it may be warranted, like in this post where people are talking about catching partners cheating who otherwise would have denied till they died. That closure is important and the trust is already gone so who cares. If I was in that position where I was so sure that I snooped and it turned out not to be true? I'd essentially treat it the same as if I was the one cheating. I'd probably rather we break up anyway because the trust is gone and it was my fault.