This is horrible, but...
In the last few weeks I've hooked up with two different girls who have given me the "I have a boyfriend line." After dealing with these girls I can only imagine how many guys have been cheated on.
Girl A says, in the car on the way to my house, "You know, if I do sleep with you tonight, you should know that I have a boyfriend and I'm a bit conflicted about whether or not I should" I just looked at her and shrugged and kept driving. Later, she said she didn't feel bad because "Everyone gets one ****up, and he already cheated on me". She dumped her boyfriend a week after sleeping with me.
Girl B is an ex-girlfriend who I have avoided for months since we broke up. I was determined never to give her a second chance, but last night I happened to run into her roommate, then her best friend, and after enough talking about her I decided I would drunk dial her for a booty call (a horrible idea, in retrospect. I was drunk and didn't care at the time though).
I said "Hey, come over. Let's hook up"
She replies "What? Are you serious?"
"yeah"
"But I have a boyfriend"
"So what. Just come over"
"You're not playin with me?"
"You comin or not?"
"Ok! But don't tell anyone!"
So, in short I've only gotten the "I have a boyfriend" line from girls that were attracted to me but resisting because of the boyfriend, and in both cases my lack of cocern over the topic was all it took to convince them.