I came searching looking for others who have had this happen to them and how they handle it.
I think the reason he left the phone out as a "test" is because I told him a few days ago that I would never look at his phone, but I couldn't help myself.
When a married man is guarding his cell phone with his life, that is a BIG sign that there is stuff on there that he doesn't want a wife see. In fact being very overprotective of his phone is the first sign that a wife will see/ remember when she thinks back to when she first suspected he was doing something shady. He may or may not be having an affair, but is at least doing some inappropriate texting/emailing/calling another/other women.
Is texting and sexting considered infidelity?
There is nothing "okay" with that. Not at all.
You deserve the best as a woman and that is no good. Confront him that it is not okay, and tell him you will not put up with it. There is no reason you should have to. Good luck to you.
All I can say is that one misstep can ruin one's life and one's family forever.
My SO thought it was cute and an ego boost to email and flirt with a girl who turned out to be underage (while I was working my butt off trying to support us). One and a half years of hell later, he is facing 6 months in jail and a life-time of having to register as a sex offender.