Dick pics

Why do men think its OK to send you dick pics and videos of themselves masterbating when they are in a relationship with some other woman? This guy I have as a Facebook friend added me on snapchat the other day and started talking a bit flirty. I told him I know he has a girlfriend so he should stop and he said its ok bc we're only chatting. Then he went on to send me pics of his dick and videos. I mean, if he was single... Its a pretty nice dick I won't lie. But he's taken!! And I'm not a boyfriend thief. It just gives me no fucking hope for any Faithfull relationship in the future