yeah, I've experienced this, and I do find it annoying when people think they can give me a hard time for saying "it". One older woman at work has taken to calling it "Baby I.T." as if it's going to come connect her printer or something.
GRRRRR! I choose to remain unaware of the sex. And, it's mine. So, bug off.
(and yeah, when I slip and say "he", because I've been convinced it's a boy by everyone who sees me, people get all excited like, "you said 'he'!! it's a boy, isn't it?" uh, might be, might not be. 50/50 chance.)