How do you know when a girls DTF?

He apparently thought I was DTF. This was from a normal small talk type of conversation, from people who haven't seen each other since they were in elementary school. No one hit on anyone, I wasn't flirting. He found my Facebook he messaged me and asked me out. Turns out all he wanted was sex and when I told him I only have sex with my BF's he basically went ghost.

How does a guy determine a girl who they think is DTF? (Do we do things without realizing?)

How do you know when a girls DTF?
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