Me and my partner aren't guarded about our phones. He often leaves his laying around, as I do mine. Which I think is healthy, as we have nothing to hide.
He isn't particularly techy, so his social media is quite primitive. He has only ever used Facebook and usual texting, I have checked throughout our relationship out of my own nosiness, as he also has admittingly done too and he once, to my delight rejected another woman's advances, who he was only casually seeing when we met, telling her he had met somebody he really liked and doesn't want to duck it up, which I did find sweet. He doesn't really do anything interesting other than sharing the odd meme/joke or funny video. He does look at the odd bit of usual free porn, I don't see it as a problem at all, we sometimes watch it together. I would however take issue if he had a subscription for only fans or simular. I think it is offensive to splash money like that on another woman, especially when there is plenty of free porn around if he wants to bash his Bishop.
Liking pictures I would take issue but in that respect he has been quite respectful like that, he told me quite early on in our relationship he wouldn't like it I I was liking mens photos so respected that too.
There was a time last year, we had a row and he went off to the pub, he got very drunk and was chatting up another woman, that we both knew and a relative of his had to intervene, which I had mentioned in another thread. It hurt and made me so insecure, for weeks afterwards I would constantly check his phone to see if they had been messaging eachother. It was a spur of the moment incident which happened in the flesh rather than anything from online flirting. It was very unhealthy of me, our relationship became a little toxic for a while, but we ended up having a lengthy discussion about clear boundaries after I knew I was becoming slightly obsessive over it all and it gave us an opportunity to air out any niggling concerns surrounding social media too, that we would have probably otherwise kept to ourselves.