There's an old saying, "you are what you eat". In internet terms I think it's true to say that "you are what you post".
Personally I'm very careful when I type, with appropriate punctuation and as few spelling mistakes as possible. I spend the extra time because I know that if I don't, people will get the impression that I'm ignorant of grammar and that I can't spell.
Since the only impression you give is the one that you type, it's totally reasonable that someone would form an opinion about you not just on what you post but also the way you go about posting it.
If it doesn't matter to you, it's not an issue.. but you can't take exception to someone drawing simplistic conclusions over something so fundamental to this medium as grammar. If you're going to take exception, fix the grammar. AKA put up or shut up.