How do I get past a cock-tease?

I've been chatting up a girl for a while now and honestly my texting game is the best it's ever been. I tended to start all my new relationships with people by trying to be the nice guy, cause I'd be nervous, who cared about the convo and tried to keep things going (too long sometimes) but would screw myself over whenever I tried to make things sexual cause they didn't see me as that kind of guy. This time I stated my clear intentions that I want to sex her, made sexual jokes and didn't take anything personally. I even used the shit test response to her "I'm seeing someone".

Here's where my question starts. What should I do with a girl that is clearly into me, responds to my sexual jokes positively, sends photos in dresses and poses but mentions she has a boyfriend and if I ask to hang out she claims "strictly friends. no funny business.".

How do I know she's into me? Apart from the photos and responding to my sexual jokes, she clearly stated she's attracted to me (I think you're good looking, you're hot, etc.). She even asked how far my uni is from her possible choice and was happy I claimed it was a 10 minute walk.

I feel like she's keeping me on the hook. I don't want to be some chump who keeps her company, entertains her and sticks around until she needs to be sexed. I bug her for having a bf and make it clear that I want her. When she asks for my opinion on something I tell her to ask her bf or that I'm not her girlfriend (I know what I said). I would never do anything with someone in a relationship but I made it clear I don't want a relationship so she doesn't dump him assuming she can date me.

We've been texting for a month (haven't seen her except once on the subway, kept my shit together, gave her a boner) and I understand there are other girls out there. However, I keep conversation going as a side task incase an opportunity arises. Should I keep conversing and hope there's an opening for her opening or just slowly kill convos and one day in the summer text her "Hows the bf?" and hope she's single?

TL;DR: Girl appears to be keeping me on the hook although she appears genuinely interested in me. What do?