If a guy ever asked me that, I would wonder two things: "Why didn't you ask me this before I started another relationship?", and "Why the f&ck are you asking me this at all?" It's a pointless question and since she has a boyfriend, it is inappropriate. And saying, "can I ask something, but you don't have to answer"... is juvenile. If you wanted to date her, you should have asked her out before she had a boyfriend. Now she has one and it's not fair to even ask her if she would be interested in dating you, because that is entirely hypothetical and just weird to ask.
"Caring is not an advantage."