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There are many girls with multiple boyfriends. If you go after girls without boyfriends then you are lowering the amount of girls you'll have intimate relationships with.
That's very true.
As for your situation being different, it isn't. It's common. She is open to the possibility of finding a better match, just like probably upwards of 70% of women in relationships (dating and marriage).
She didn't bother to mention her bf to you, for fear of scaring off someone that she thinks fits the profile. YOU'RE a better match and one that she wants to give an opportunity, so don't let it pass you by.
This doesn't mean that it is even necissarily a conscious intention. There are many cases where a girl says that she is happy and convinces herself that she is and that she will continue to be with the guy she's with, even though she's really not happy at all and wants a way out subconsciously. I have no problem "rescuing" those girls, it's what remains of the AFC in me.
Like you said, the only girls that you can bet are in a happy relationship, are the ones that are saying good things about their bfs, mixed with a little bit of normal complaints, but not so much to either side that she's complaining all the time, or that she's saying so much good stuff that it sounds like she's over compensating to convince herself he's "the one". It's definitely not something that you can apply a forumla too, because there are far far too many variables and in the end it all comes down to your own morals and judgements. It's a basic guideline at best, but it's the best I've got.
Like Tripp said, in this case, I'd say that you should go for it.