I've asked some girls out randomly and have gotten the 'boyfriend' reply. Now, do they REALLY have one? Was I 'not their type'? Were they too nervous or caught off guard? Who knows. What I have found that works...or rather injects some humor into a potentially humiliating experience is saying (after they say they have a boyfriend) "Well, I'm not asking to (f-ing) marry me/I'm not proposing to you, I want to know if you'd like to hang out"

If the girl is REALLY not interested or has a very serious boyfriend, they are still going to refuse. But...they've found it humorous enough that I didn't feel like a tool after. This approach has worked for me 3/5 times I've used it. Two of those three girls liked the relationship they were in enough to not progress past the first 'date' but told me they were very impressed with my confidence and that alone caused them to accept. The other one was in a relationship but it had become very stale. In the end, we both decided that we weren't a good fit anyway.

So, if you REALLY are into the girl this might be worth trying. Like I said, at worse you might make her laugh.