But it is well within your right to ask to see the phone.
Also, if what you see bothers you, then the bigger question is to ask.
What needs does this person fill for you.
If anger because couples privilege is violated, then listen first talk later. Then ask your self am I neglecting him in some way
For guys .... my question always is, when was the last time you gave your partner a hug with no intention of sex.
Accessing private thoughts without consent is violation of the mind.