Girlfriends phone habits and ex lovers.

My girlfriend and I have been together for 1.5yrs and all is great. Her three kids get along great with my two. I sold my house and moved in with her. But earlier in the relationship, ie, first couple months. I started noticing her sneaky phone habits. When I would come into the room, she would be quickly closing aps until she was at the homescreen when I got close to her. Or she would get text messages and not look at them until she was alone or could angle her screen. On one particular night in the first couple months it was particularly obvious so when I got the chance i looked at her phone (wrong to do but something wasn't right). When I looked of course I saw her saying to her girlfriend that she might sleep with a Co worker from her last job. They had always had some sexual tension between each other. I confronted her on this and we eventually worked it out. Although she was over the top pissed that I looked at her phone and never took any responsibility for her actions in making me feel like I needed to look or what she was planning. So six months later and no change in her sneaky phone habits. One night again I had had enough. So I looked again. No surprise to what I found. This time there were several things. First off, her first sexual experience after leaving her marriage was to this guy she sketched(oh ya... She's an artist). She said she sketched him about five times before I came into the picture. Since I came into her life this guy has repeatedly asked her to sketch him. I told her that I didn't want her to because of their sexual history. She said it wasn't a sexual thing... Just art. Yet in his text to her all he kept saying was jacking off on her tits. So yes it is very sexual. She eventually agrees to sketch him secretly, but not in her house because it could lead to cheating. So she could cheat. In another exs text he tells her that his penis misses her and her reply was... "not a singular desire, mutual". Their sexual history is more troubling to me because it had a definite fifty Shades of Grey angle sexually. So I'm certainly concerned about any sneaky hookups. My only way to verify is by looking at her phone. If I talk to her she will just become better at hiding it. I need to see that it isn't going anywhere. I know I should leave but our kids get along well and we are good together, in all ways. She just can't let go of her past. Also, her best friend is a guy she dated for six months then she dumped. He has no kids and no interest in hers. They get along almost to well and yet sexual I know they don't. However, she is constantly asking him to go away on a quick get away. In a b&b or a cabin. I have met him. When ever she is talking to him on the phone she sounds like a high school girl. She goes to his house a couple times a month for dinner and wine and stays from 4-6hrs. I'm close to the end with all of it but damn our kids get along so well but when ever she's AWOL from the phone for hrs I'm suspicious.