Is this a normal thing men do?

This guy online had been texting me and flirting with me and sent nude photos to me. He was said to have liked me or whatever. We never met in person. I found out he has a girlfriend because he posted it, some what sneakily. My question is, is that cheating on her? Is it normal for men to text woman behind there back. They are still together. They are long distance too. I just can’t understand if she knows he text other females and it’s just not a big deal to her. Lol I’m new to relationships and I’m just genuinely confused. I can’t understand why he would send nudes to a female online while being with another woman. Is it just for fun on the side, and she’s really who he wants to be with orrrr?