I agree with both sides here, and as much as I hate to sit on the fence, this is an issue that is never going to be truly resolved either way, so it's best just to argue both ways.
I think that first of all, going after a girl with a BF isn't necessarily a "sad" thing, because as much as we all hate it, we don't control who we are attracted to, and that includes girls with BFs sometimes. They don't all have "BF" stamped on their foreheads, and if you don't know until after that attraction has developed, then you are stuck in a bit of a predicament. Are you going to just tell yourself to stop being attracted to this girl? Again, it's not something we can just flip on and off like a light switch. The attraction will likely be there, but whether you act on it is another story. It's difficult to just pack your bags and move on so easily because as many HBs out there, every one you meet there is a possibility that they're attached. As far as I'm concerned, I refuse to go to a bar to meet women, because I'm not looking for that type.
Another seemingly overlooked point on this board is that it's always easy to look at another person's life and say you'd do this and that, but once you're in that position, you see that it's easier said than done.
I have been attracted to girls with BFs, and I think it's ok as long as you limit it to flirting and social proof while they're attached (no harm, no foul here). If they find themselves attracted to you enough to dump their BFs, then it's ok to make a move. Spending all your time looking for unattached girls that meet your criteria isn't as easy as it seems, and if you've found a great girl who is attached, don't just next her, keep her around while you're searching. No harm here. I mean, you can know a girl for quite a while and never know she has a BF until you've known her for a long time. I knew this girl for 3 years before she even mentioned her BF, and I never really saw her as more than a friend.
As for the flip side, if a girl cheats on her BF with you, that's a problem and you should bail out immediately. Make sure she dumps the chump before you do anything with her. Sometimes a girl comes along, and she's absolutely perfect for you, and if she's attached, it's very hard to just never see her again, especially if you've known her for a long time.