My partner keeps lying

My partner reckons the reason he has porn in his activity log is because you have to search it, just to delete old history from years ago. Does he seriously think I’m stupid? Hes lying right? It’s not the porn that upsets me, it’s the lies. So if someone could give me some tips on why he lies to me all the time? (Btw) he’s also cheated on me in the past, and has lied several times.