The boyfriend objection is when a girl says "I have a boyfriend." The boyfriend may or may not actually exist. According to The Shit Test Encyclopedia, the correct translation of this phrase is, "I have Schrödinger’s boyfriend, demonstrate to me you’re high value and I'll fuck you regardless."
Motives for using this objection
An unemotional "I have a BF" response can also be a way of telling you to fuck off. On the other hand, if the situation has been escalating up till this point, it's probably last minute resistance.
Part of her goal may be to avoid responsibility for what she's about to do, since she can think to herself, "I put up resistance by telling him I had a boyfriend, but he went ahead and seduced me anyway." It can also be a way of telling you that she wants alpha fucks with no strings attached. It can also be a way of telling you that she needs you to be discreet (so that she doesn't get a reputation as a slut).
Possible responses to this objection
"Good! He'll listen to your emotional problems and take you to romantic dinners, while I'll use you for sex/fun/[whatever you find appropriate that alludes to sex]."
"Damn, I've only known you x mins/hrs and already you're telling me your problems."
"Oh, so when's the wedding?" Usually the shock of that question will help loosen the mood and show how serious she is about her SO or if she's just using it as an excuse to get you to fuck off.
"Where's he at?" "(Not here)" "Aww, how come?"
"Don't worry, I'm not the jealous type."
"What's that have to do with me?"
"I won't tell if you don't."
"Sorry, he's not invited."
"I know." (Kisses passionately)
(Ignores it altogether and continues escalating)
HB10: "Oh did I mention I have a boyfriend/husband?" 1026: "Yeah, you did. Like 5 seconds ago" *continues to plough ahead*