Guy in my class is flirting with me but I look up his Facebook and he has a girlfriend? What's going on?

I sit next to this guy in one of my classes. He's very attractive and very nice. We usually chat before class and stuff. We are also doing a semester-long group project together (with a few other people) so we exchanged numbers. A week ago, out of the blue, he texted me and said that he hoped I had a nice day. I was very confused, wondering if this was flirting or if he was just really friendly.

So I respond back, and every since then we've been texting. I've never instigated the texting, it's always been him. He texts me after our class together and tells me he hopes that I have a nice day. He's told me that I looked nice, sent a couple winky faces, and some other various flirty texts over the weekend. All signs would say that he likes me or he's interested.

But then I go look him up on Facebook, and there's another girl in his picture. Turns out, she's his girlfriend. When I click on her profile, it says she's in a relationship with him. I am beyond confused. I feel really uncomfortable now knowing some of the flirty things he's said to me.

I see a couple of potential things going on here. They either broke up and never changed their profile pics or status, he's just abnormally friendly, or he's sort of cheating on his girlfriend with me.

What am I supposed to do about this? I don't want to be the "other girl" and I'm not sure I want any sort of romantic relationship with this guy.
He's just really friendly
Vote A
He & his gf never got around to updating their pics and status
Vote B
He's cheating on his girlfriend
Vote C
Select age and gender to cast your vote:
+1 y
I would also like to add that by looking through some of their photos, they've been dating for at least 2 years. I don't flirt back, but if I didn't know he had a girlfriend, I would 100% think he was interested and going to make some sort of mood. I just feel awkward now.
+1 y
Guy in my class is flirting with me but I look up his Facebook and he has a girlfriend? What's going on?
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