Happened to me multiple times.
I also learned that if you're angry about it, you become a problem and realize later that maybe they never were a friend to begin with.
To expect sex at cons or from friends is asking for dissapointment.
I have no obligation to sleep with anyone just because we like each others company.
Its even worse when you are guilt tripped about it.
Fandoms weird. You admire someone, You want to talk to them.
You attempt to. Say hi and make small talk.
Never, EVER EVER speak to each other again understanding that they have no interest in you after the short interaction.
You silently avoid and ignore them going forward.