Why does he go through his 'friends' facebook pictures so often?

My boyfriend of almost two years goes on his facebook and goes to a girls profile to literally look at every single one of her pictures. I asked him about it because he did it over three times already. His excuse: She’s a professional model so what if I look? (cough) she’s 5’2 working at banana republic, she isn’t a model. The next time he said : I don’t know why that is my history. Really? Is he masturbating to her photos? or some shit? He deleted her. but searched her up again and said my computers flipping out, i dont know how it got there. He works most of the day and only has 4~5 hours of sleep but he has time to go on youtube and watch girls prance around in their bikinis? I dont get that either. If i did anything close to what he does, he’d flip. But when he does it, he say, I HAVE NO IDEA HOW IT GOT ON MY HISTORY. TRUST ME OR DONT

If he watched porn, i wouldnt mine, just say so. But looking at people he knew from real life and going on youtube to watch girls prance? what is that? It honestly disgusts me. This then raises a question, why does he lie so frequently? Why is it when i try to talk to him about it he gets frustrated?