SO lies even when I know he is.

My SO keeps lying about the littlest things and it's frustrating. The other day I got on his phone when I got on safari it was already on his history. So I scroll down and find porn websites. No biggy. So I casually ask him if he had watched porn recently. And he says he hasn't. So I show him the phone history and he says he doesn't know how that's there. So He adventually tells me he did but he didn't want to make me mad. I've told him a million times I don't care so why lie. He does this all the time about the littlest things? Any advice? Is porn that embarrassing for guys to admit to watching?