Long time lurker/reader. Thanks to TRP for showing me the light.

Problem: I know I know, don't shit where you eat. HB9 (mid 20s) in a work circle enters the picture. She has a bf, multi-year LTR, but seems miserable. Immediately begins flirting with me. I get her out in a neutral setting and try to escalate after many IOIs. The logistics went bad, called it a night. The following week, she's all over me at work, so much so that it starts drama with other females. We hang out the following weekend. I escalate and try to make out, she doesn't respond. She's smiling, saying she can't cause she has a bf, etc etc, typically ASD.

My response was to simply tell her I don't need more friends and the only reason we are hanging out is because I'm into her. She assures me the feeling is mutual and she's just trying to be "cautious". After that I assumed all is lost and I'm just some fun attention, ok, now I know on to the next one.

Then, she keeps up with the flirting at work, which we both agreed should cool down, initiates texts, constantly makes herself available, asks me to hang out, talks about sexual things, (like telling me she would have already slept with me but for the bf, more ASD). Things that someone without interest wouldn't be doing.

It is entirely possible she doesn't know what she wants and doesn't know what to do. Also entirely possible she's just playing me for the attention. I mean I have likely spent more time with her the last few weeks than her bf. Also a possibility that she's slotting me into backup #1 position. Where do I go from here?

EDIT: Also as a side note, BF is definitely low SMV.