I think there is a huge difference between an employer regularly checking internet activity (I have been in jobs where they have checked activity too, this is expected now) and checking to see whether a text is from work or an elderly parent or whether it is 'Ms. X who just wants to say 'Hi') therefore enabling you to decide whether a partner needs to be woken or not AND reading correspondence as rote because you want to. If I send a text to someone to their private phone, I don't expect them to share it, doesn't mean someone can't SEE it, it doesn't mean that I don't realise that internet correspondence is not entirely private, but I should be able to trust that if I write someone to someone they are not sharing it around without telling me (I don't care if that person is their spouse, mother, carer, probation officer whatever) I should know that I should not have the expectation that that correspondence is private.
And if I know something is to be shared it would definitely inhibit the way I write, having been in that situation more than once (it was awful) I would not allow myself to be treated that way again. I hope all these 'I am entitled to read anything I want' types are honest to the people they are involved with that this IS something that happens so they can choose whether or not they want their privacy violated like that.