Don't pay those ignorant asses any attention they aren't educated enough to come up with an original or well thought out insult. 'bottum' is all they have- amongst 'phagget' 'ghey rod' and other generic played out insults.
Yeah, I find this weird, too. It's just because it's not the masculine role. There's nothing wrong with being a bottom. The tops would die without them.
Seriously though, this topic bugs me a little. My best friend is a total top and he's always said before if he ever had a gay son that there's no way his son would EVER be a bottom. I've argued with him about that because he was talking about it like it was such a bad thing to be a bottom. It's preference, that's all. Let people be as they are. He just couldn't bear the thought of his son getting plowed and/or used by another man. Because then he'd be less of one, or something.