I came up with this not on the spot last Friday but as I was walking away thinking of the best possible thing I could have said to the "I have a boyfriend" line. Which I was gutted to get fed on Friday from someone who was sadly exactly my type.

Here goes, remember this is genuine for me, especially since my last two serious relationships were with girls who broke up with their boyfriends for me. Anyway...

" I have a boyfriend"

" You know thats too bad,

Not because you have a boyfriend, but because it means we cant get to know each other. It wouldn't be fair on you.

You know I'm not good for girls in relationships, Ive met many girls in my time and when their just with someone and Ive decided not to pursue it and just be friends. Literally after a month or so they'll end up breaking up with whoever their with. *gesture over her shoulder*

Its like, you know when your with someone *gesture to her* and its just safe and you just don't want to end being on your own even though it could be so much better?
Then all of a sudden you meet someone new and exciting *gesture to self* and you just see all the fun things you really want that your missing out on in life, it just makes you questions things"
*pause*
I just don't want to do that to another person, sorry"

Then walk away....

If she's in a secure relationship then she wont come after you, but if she's looking for something more then you've just demonstrated your the person to give it to her and she'll stop you.