My secretary is hitting on me, and I have a girlfriend. What do I do?

My secretary who is currently 24 keeps flirting with me while we're passing alone in the hall way, or if my assistant is not in my office with me. She hasn't made any physical attempts, but has comments about my recent working out, good looks, how she wished she had a guy like me, and even admitted that she wanted to sleep with me at a work outing. Of course; I resistsd all of her attempts; and made sure to tell her that she's acting inappropriate.
After a month or so of her flirting, and me reminding her that it's not ok for her to do this. I had to make the decision on whether or not to approach HR, and my boss as well. Since she's very much a valued employee at our company, and has been the best secretary I've haved yet. It's been difficult making the decision to let her go.
So now we come down to what I need help with. After my discussion with my boss, and HR. They both decided that her work outweighs her advances towards me at this time. Since we've had such horrible luck with secretary's at our company before. Both were reluctant to let her go. HR wanted more evidence, and since its ran by a high strung feminist. I actually received some backlash about blaming a woman for hitting on me, and that I must have been encouraging my secretary to thinking that I like her.
Now I'm stuck with the HR lady trying to look at me as a womanizer, my boss who resists confrontation at all costs, and a secretary (who I forgot to mention is highly highly attractive) who keeps wearing tighter, and tighter outfits.
I truly don't want to quit my job, but I'm in a tough spot.
My secretary is hitting on me, and I have a girlfriend. What do I do?
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