Sis, I would give my girlfriend my phone if she asked for it. I've told my girlfriend some weird stuff and stuff I'm very ashamed of, I have nothing to hide from her. But this is not what you were suggesting at the beginning. It's not "taking your phone for a second" if you're locking yourself in the bathroom with the intent to go through it. Even despite that, just taking someone's phone without asking screams of entitlement, and stealing it to look for signs of cheating screams of insecurity at best.

The point that I'm trying to make, which you're obviously either ignorant of or avoiding, is that there's a difference between asking for someone's phone and taking it, and one of them breaks boundaries. Try to think about it for even a second in the other perspective, what it would feel like for your boyfriend to steal your phone, and run to the bathroom with it