...So their existence is offensive to themselves? No, actually, most muggles don't mind being muggle. And I've yet to meet a muggleborn that minds existing. An insult is something that offends someone. I'm not offending me. You are. Though not for the reason you'll probably assume.

For example, your stupidity is not offensive to you (presumably), if only because you've grown accustomed to it. So therefore you are not an insult to yourself. You are, however, a very grave insult to your house, who do not wish to be seen as the house of fools. And your housemates know where you sleep.