If she's talking sexual with you, she might be willing but first testing to see just how easy you are. It's quite easy to play this game. Joke with her and keep her guessing... Best thing to do is to playfully reject her.
Some examples:
"Hmm, I donno if that's a good thing to be talking about... Don't want you to be wetting your panties at work."
"And why would I want that? Friends don't do that!

"
"I could be the best... But not like you'll ever experience it

"
Eitherway, two rules:
1) Never outright admit you're DTF
2) Never be serious in your conversation... Exception: you're talking about sex with a previous gf (NOT A BUNCH OF GIRLS) and how hot it was by giving concrete examples without going into details (e.g. you fucked in the park at midnight). This is to make her want to experience it with you.
Playfully rejecting her will only make her want you more.