I'm curious what made you want to check her phone? Did you already have trust issues? Have there been instances of flirtation that made you mistrust her? The fact that you went through her phone says there was already mistrust on your part. I am not going to condemn you for that because I've gone through my husband's phone withotu asking and did find flirtatious texts, as I had suspected.

The other question I have is how could she say she doesn't remember sending sexy half nude pics to this guy? Of course she remembers! We all remember who we send sexy pics to, unless she's sent these kinds of photos to every male "friend" and cannot remember who or when. I think it's fishy as well that she can't "remember" sending them, which would send my own alarm bells off. It seems she's not exactly owning up here.

I don't think it means you need to break up necessarily, but I would have a serious heart to heart with her about trust, and about whether you can trust each other to not be seeking out attention from the opposite sex.