What does it mean when a girl has a boyfriend but flirts with you and sends you sexy photos?

I meet this girl in a class at the start of the semester, after a couple of classes I decided to ask her out and we connected instantly. It was crazy, our first date ended with our first kiss and we had to pull ourselves away from each other; but she wants to get married asap and I want to take my time to get to know each other more. Then on our 2nd date on her birthday the next night we had a great time and eventually hooked up. Then after that there was a weird distance, so I told her we should see other people, but after a couple of weeks and a couple of dates with other girls I realized I was still stuck on this girl. So I told her how I felt and she texted me back and we hit it off again and even did some flirting and light sexting. Though when we're in public she's kind of distant but when we're alone we have a good time. It's been going on and off for a couple of weeks now. But I have no idea what to do cause I really like this girl, but only known her for a month and hardly know her, she never once said she wouldn't want to date me or anything just that she's with a guy now. I'm stuck between flirting and light sexting with a friend which would be like the weirdest friend zone ever or just moving on cause I don't think I'd want to put that much effort into having two "things" I don't know I've never thought about having this kind of things going on so I'm a bit confused. Any advice would do haha
What does it mean when a girl has a boyfriend but flirts with you and sends you sexy photos?
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