OP, you should be glad your previous date was honest with you that she was unavailable. Of course it’s also possibly there is no boyfriend but that would mean she was so unimpressed with you that she would use an excuse to avoid you… Either way the lady has given you the message that she’s not interested. Take that to heart and look elsewhere. I seriously doubt the lack of interest was because you didn’t “make a move on her”. First dates are just about getting to know each other well enough to decide if there is potential for a relationship. Did you by chance start talking about politics? Were you rude or condescending to the wait staff? Did you spend most of your conversation talking about yourself and trying to impress the lady? Those are really big turn offs for most people. Every conversation should have as much listening as talking…

You sound like a pretty promising side piece, except you're wasting her time with the "taking it slow" bullshit. She's not looking for marriage material -- she already has that. She just wants to bone.