I'm almost certain this person was signed up on BL under a different name, asking the exact same questions. He was obsessed with why a girl might seem interested, and then suddenly not. He also kept referring to himself as innocent.
Either way, doesn't matter, it sounds to me like this "innocence" you seem to think you have is actually your insecure desperation, and you're comparing this seemingly harmless personality to most of the other egotistical, douche-baggy personalities all the other guys at the bar seem to have, and wondering why women aren't attracted to you. While yes, at first evaluation, you would think a girl would be much more inclined to talk to someone who isn't a chest-pounding dick, someone who comes off as non-threatening, soft, welcoming, and interested in meeting a potential girlfriend and not just a quick lay....all things women fall over themselves for, right? What more could a woman want than this breath of fresh air in this meet-market of testosterone?? I've been there, it's an easy mistake to make, but as you're realizing, this is a poor approach. I'm not saying be fake, or be douchy on purpose, but it definitely sounds like the essence you give off is....well...sorry, but.....pussy-ish....and yes, a woman will pick up on that in minutes if not seconds, and chances are she's done with you. The boyfriend statement, while it could be true, is very likely a quick red-button exit stage left excuse.
Somewhere I can suggest you start, pretend you have a girlfriend already and you're just kickin' it at the local bar either with your homies just to have a beer or two before you go home to your gorgeous girlfriend laying in your bed waiting for you, and aren't there with the purpose of meeting someone. Think about how you would feel if this was true, and try to evaluate the confidence you would project. Try to project a similar confidence, and see how that changes things for you. I have been with my fiancee for over 8 years, and at this point, I can't get all the beautiful women to STOP talking to me, flirting, some even have suggested that I "adventurize". I think back to when I was single, and if I had this confidence level back then, I would've gotten half the population pregnant.