STORIES.
I could write a book with stories about girls I've met on PlentyOfFish.
Look at me compared to 99% of the guys on there. If you're In-Shape, have social skills, and not creepy or think it's cool to be a dick, is like fish in a bucket.
... Now back to the original topic.
Like I said earlier, most gay guys know if your gay or not. Same way as a straight guy, we can usually tell when something is "off" about a guy. Usually we tell by a dudes mannerisms, or way of speaking (look at Sergio Oliva Jr) , but I'd assume gay guys might notice eye-contact n shit too... Maybe grooming or how a dudes dressed. Eyebrows plucked or beard trimmed to beat maybe. I dunno.
So IME, gay guys that hit on straight dudes usually do it to see how they'd react and get off on making the dude uncomfortable. Same way a pervert or flashers gets off on making a girl uncomfortable