Cheating or not ?

Okay guys, so I have a bf who I have been dating for like a month now . And this is the first time ever I've ever gone out with a 'guy'. So yesterday I thought I would suprise him since it's been a month since we started dating ( yes it's really childish but I was just planning on visiting him and just spend some time ), so his mom let's me in and then I open the door to see him masturbating to this girls pic who's from my class. So I just went back home. When I asked him about it he said it's just normal for guys to masturbate and that it's like my periods. So my question is would this be considered cheating ? Am I being too possessive? Also would him masturbating to porn be considered cheating ? It might be an obvious for other but I've only dated girls till now, so plz gimme some advice (〜￣△￣)〜