Thanks for all the responses.
But I still believe that what people write and what happens in reality are sometimes different. People try to adhere to certain moral ideals but sometimes in a particular situation it's not so clear-cut.
In reality it's hard to break up with someone without having someone else to go to for the following reason. If a woman decides she wants to move on and get out of a relationship, after a few days or weeks she gets lonely and misses the good things about the boyfriend. Inevitably, she'll go back with the guy to try to "make it work" once again. We've all seen relationships like this, that they break up several times before it's final.
That's why I'm saying if a woman seems interested in a guy but says she has a boyfriend, it doesn't mean that she is not be interested. It could just mean she's never had someone to go to after the break up.
And I understand the argument that I wouldn't want to be with someone that can't be single or that needs to cheat, etc. It just seems like many women are like this, so I don't think it's too abnormal. I don't know if I can hold it against someone. In fact, many people would hold it against someone if he's have been single for too long.