Not every girl on the street is with a guy. I personally won't approach a girl with a guy as I too assume he is her BF. It's just not worth it. I see girls out by themselves or with groups of girls more often than girls with guys. Or maybe it's that I immediately disqualify and make invisible, girls who are with guys? Not sure.
You don't have to small talk with girls. I don't. I actually have very deep conversations with women - so deep that it really turns them on. They feel like this is their Cinderella moment where they've met the man of their dreams because they've just had the deepest and most profound and personal conversation with someone who really sees them and is curious about them. It plays very well into their love at first site fairy tail script, but all that jaded commentary aside, there's nothing fake about what I'm doing and I am 100% sincere in my being curious about them. I've just learned how to have deep, fun, and meaningful interactions with strangers instead of small talk. I wasn't a natural at it. Not at all. It's like nearly everything else: something one has to learn and practice. I've spoken about it many times on this forum and given specific instructions on exactly how to do it. It's really not difficult actually. But most people read it and think, "Oh, well that sounds easy. I know I could do that" but then they never go out and actually apply it.