This has happened to me only once. When I asked the insanely hawt big-boobded bar wench out, she said she had a boyfriend.
She might have even been telling the truth, but as Voltaire said, "If the insanely hawt big-boobded bar wench didn't have a boyfriend, she would be forced to invent him."
This was two years ago, so she might have gone through a few boyfriends since then. Strangely, she's the only woman I even encounter on any regular basis, so I should ask her again. Unfortunately, there's really nobody at the bar with whom I would feel comfortable discussing her availability status.
Lo, I am become Domindart, creeper of bar wenches!