All you're really saying is, Warning: I'm a prick and I'm about to prove it.
Someone in my family works with someone who prefaces a lot of statements with, "You know I'm not racist, but *insert racist statement here*." Actually, yes, yes you are racist. Just because you claim not to be before making some ridiculous racial judgment, does not negate the racial judgment.
The same goes for statements beginning with, "I'm not an asshole but *insert super ass hat comment here*." Fuck you. Actually you are, in fact, an asshole. Not because you said that, but because you made a bold disclaimer about how you know you're being an asshole.
The irony is, if you make an asshole comment without starting a sentence that way, you could turn around and claim that you didn't realize you were being an asshole. In which case, people may be more forgiving. But by calling it out before you even make your statement, you are waiving all rights to being forgiven easily, because you are taking ownership that you know your comment is fucking bull shit, before you even say it.
If you feel that a decision you are making, or something you are saying to someone, requires a disclaimer about how you know it's a mean thing to do or say, or that you have to start it with, no offense but, perhaps you should reconsider what you're about to say. Are you about to hurt someone's feelings? Are you about to make your friend angry? Might you not need to really say what you're about to say? What good is about to come from what you are about to say, knowing full well that it is an asshole comment?
Don't get me wrong, I am all for honesty. Speak your truth. Be real. Do what you need to do. Just be aware of how fucking idiotic you come across when you start a sentence with the admittance of what a jerk you are. And know that as that sentence comes out of your mouth, you are about to say something that will most likely hurt the person on the receiving end. And know that after admitting that you are about to be an asshole, and that after you make the statement anyway, the worst thing you could follow up with is, "I'm sorry."
No, you are not sorry. If you were sorry, you would not have made the hurtful statement in the first place. If you were sorry, the outcome of this conversation would be different. If you were sorry, the person you just hurt - knowing ahead of time you would hurt them - would not be hurt. Because if you felt any sort of actual remorse, the conversation would have gone much differently. So don't say you're sorry when you're not sorry. That's even more annoying. "I don't want to be an asshole, but *asshole comment*...but I am really sorry" is literally the most annoying group of words a person could say out loud. Just shut up instead and say nothing.
I don't want to be rude, but...