What’s sad is that I’ve even been asked things like this by women
who have complained in the past about being asked these questions themselves!
I agree with Scottgun, though; I think it’s generally not intended to be malicious–they just speak before they think. And I have said some stupid and/or thoughtless things in the past without thinking, too, so I try to give them the benefit of the doubt. I am polite in answering (or “non-answering,” as the case may be). At least ONE of us should try to remember our manners, right? And since they asked the question, I guess it’s up to me now!
"Are you pregnant again?" - I haven’t been asked this one by anyone except my husband (and not in a negative way). I would probably be okay with answering it as long as it wasn’t said in a condescending tone. If it was, I’d probably just pointedly ignore the question, or look at them a good long while with a blank stare and then change the subject.
"You didn’t have a glass of wine. I guess you’re pregnant!" - I rarely drink alcohol, anyway (maybe 2-3 times a year), so no one’s said this to me. If it was friends, I’d probably jokingly reply, “Wow, am I that predictable?”
"Was your pregnancy planned…?" - I agree with Pax2u, my first inclination is to think, “What kind of a question is that?” I think this is one of the most thoughtless questions that can be asked of a parent (expectant or not), so I usually don’t answer this one–I have to assume they just aren’t thinking. So I give a non-answer, like, “We’re very blessed,” or change the subject as though I didn’t hear them. I haven’t had anyone repeat this question.
"Are you going to have more…?" - We have seven children, and I get this one (and similar ones)
all the time! Yet I’m still not certain how to answer, because I’m not really sure why they’re even asking!

I usually say something like, “We’ll see what God has planned.”