Funny... I never get this problem... Actually I can't remember one instance where I get the 'I have a boyfriend' or 'Let's just be friends'. (Maybe I'm too ugly to be considered a threat! )

If I did... I would say the ol' classic:

MB: Does he treat you well?
Her: Yeah!
MB: Good, cos I wouldn't! (With my standard brand of cheeky tone)

Usually she's having too much fun with me to even think about her boyfriend!

The only thing I would say, if you get this very early on in the conversation then you've given too much of a pick up vibe... try being more fun and you'll find that shit doesn't happen!