But if they have both have their own phones why would they need to snoop on the others. It's called trust.
I agree but not all men are trustworthy when it comes to sex. Not all women, either, but it seems to be a bigger problem for men.
My husband doesn't have a pass code on his phone but I'd never pick it up and view his messages, emails, etc on it.
And if he all of a sudden locked it and didn't offer you the pass code, what would you think?
This happened to a woman I know and her husband explained that he had sensitive work messages/emails on his phone and was afraid if he lost it, someone else would have access to that information. A year later, he asked for a divorce. During the divorce, she found out he had started seeing the other woman 1 day before he locked the phone.
We do sometimes briefly trade phones to share a funny story or picture, but I'd no more snoop on his phone than I'd sit at his computer and look at his email there or open any snail mail he happened to get. Trust.
Not all women are as trusting as you are nor should they be. Lots of women have reason not to trust their partners. Sad, but true. Looks like you found one of the good ones, though, but not all women are so lucky. I could tell you stories about what women I know in real life have had to go through that are more shocking than fictional stories. Like the one who found out about her husband's secret other family. Yep, thought that stuff only happened in books, but nope, it really does happen.
Personally, I agree with you. If you can't trust them, you shouldn't be with them. But things get complicated once children are in the picture and it is not so easy to leave for some women. Their and their children's standard of living takes a hit and if they have to work outside the home and share custody, it means significantly less parenting time than they would like with their children.
Obviously went off topic here, but the bottom line is this - Apple will not allow an app with the functionality the OP is looking for to be sold in the Apple app store. If you want that sort of functionality and to keep your iPhone, you need to jailbreak it. If your iPhone has already been updated to 6.1.3 and can't be jailbroken, buy a used iPhone that is running 6.1.2 or earlier. Or give up your iPhone and move to an Android phone. Those are the options.