“None of your goddamned business” is quite unsubtle.
(Yes, I know it’s your mother-in-law, but if she hasn’t gotten the clue by now, I say give her both barrels.)
I didn’t think a shocked “I beg your pardon!” at the indignity of being asked such a personal question was subtle. But I would suggest that in that case, “That’s none of your business” repeated frequently would be most useful, as any snarky responses might just be thought of as an offbeat sense of humor. For your mother-in-law, I recommend tomndebb’s suggestions. My husband and I are in our mid-30s, and have no children. When we’ve been questioned, we’ve either said we don’t intend to and changed the subject, or if pressed on it, said it’s not up for discussion. After his parents persisted a few times, he told them that frankly, being nagged about it makes it even less likely we’ll feel favorably about having kids, and that the matter was closed.
Given (a), how about, “Because it’ll kill me. Do you want me to die? Hunh? DO YOU?!?!?!?!”
I think the Thread title is a good response. In a similar vein:
We’d love to, but we can’t figure out how it happened the first time.
Unfortunately, we had to steal the last one, and we don’t want to risk it again.
I’m waiting until I’m 70 so I can make the Guinness Book of World Records.
“Well, I’d like to, but since the last one chewed it’s way out…”
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I consider my husband my best friend and you don’t expect me to have sex with my friends, do you?!?
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I already have two children at home, and one of them I’m fucking. Now you wanna make it three?!?
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Now that the kid’s sleeping through the night and my husband’s not been noticing the salt peter in his food you want me to sacrifice my sleep? WHAT KIND OF MONSTER ARE YOU?
I did, for six years, and had to fend off similar intrusions.
Believe me, he has. The information only stays in her system for a finite amount of time, which means that she raises the subject every couple of months. That’s why we are considering shock tactics, like the impotence answer or possibly one of the more extreme suggestions here. 
You know what? I’m used to this. At this point it can almost roll off my back. But there is no Hell cruel enough for people who make comments to the children themselves.
I offer you two of my “favorites”:
[ol]
[li]“Don’t you want a little brother or sister?”[/li][li]“You need to sleep through the night better or mommy and daddy will never make you a little brother or sister!” (Yes, a woman I met at the park said this to my daughter. I wish I could say that I was quick enough on my feet to come up with a clever reply at that moment, but I was just stunned.)[/li][/ol]
Fuck you! MrValley and I cannot have another child without adoption or IVF, so that second one’s particularly cruel to her. But even if we could, what the fuck makes people think that it’s in any way appropriate to try to involve a child in an adult reproductive decision? Not to mention possibly make them feel responsible for the outcome?
Here’s my bonus reply for you. If you ever get this straight line, promise me you’ll use it:
Q: “When are you going to have another one? If you only have one she’ll end up self-centered with no social skills.”
A: “So you’re an only child?” or “What’s your excuse?”
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Admit it: you wanna watch.
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I’ll supply the egg, he’ll supply the sperm, but you have to supply the uterus.
I like what one Doper once posted (can’t remember who it was, unfortunately). It was something like, “Well, each time someone asks me when I’m planning on having a(nother) kid, I add a year onto the time before I’ll have one. At this point, I’m going to be at least 76 years old!”
Bravo Lily! I’m sure I’ll have the chance sooner or later.
Before you all home in on the MIL - she is the least of it. A complete lost cause and water off a duck’s back for me. Neither do I mind having discussing the issue when the person I’m talking to is willing to listen to my point of view.
What really pisses me off are people who don’t know me, or anything about me, because it’s none of their business and for all they know it could be a deeply painful subject. Even worse are those people who really should know better, like my son’s paediatrician the other day, who was, in fact, the inspiration for this thread.
She asked me whether I was planning to have another baby, and when I said no she tut-tutted and said “that’s not good”. Rather than have a confrontation with her I changed the subject. I later decided it was time to change doctors, and wondered whether I should inform her of the decision and tell her why, or just move on. IMHO fodder, I know.
Oh, I would definitely tell her why. She needs to know that’s a boundary she shouldn’t be crossing in her practice. If she takes it to heart you might save any number of other women from the same annoyance.
OK, thanks a lot! I almost shot dinner out of my nose!
"Well, I’d love to, but for that you have to have been born a woman
What about: “When the price of veal gets high enough to pay for the hospital stay.”
“Great-Uncle Snodworthy’s will stipulates that his estate is to be split only among his childless relatives.” What bothers me most about this one is that I’d wager long odds that the majority of the asshats who ask the OP’s sort of question would accept this as a legit reason.
“We have been cautioned by the Society’s genealogists that if we have a child it will be the Anti-Christ.” Or the rebirth of Hillary Clinton.
Someone started badgering my husband about this shortly after we were told to stop trying to conceive due to my health (and shortly after he and his wife just had a baby). My husband immediately launched into a detailed description of why everyone should have a Rottweiller and how selfish and stupid people who don’t have one are. It was a beautiful thing. He never brought it up to us again.
I’ve been known to come back with a few different responses, depending on my mood. If it’s someone I know well or are comfortable with, I just explain that we can’t have children and leave it at that. If it’s a stranger at a party or something and I’m in a pissy mood, I’ll sometimes say something like “Oh, I get it. And when we’re done discussing MY reproductive system, it will be YOUR turn, right?” or “Please don’t wish that on me- I delight in sleeping until noon.” or “My husband only fucks his girlfriend.” Sometimes I just stare blankly and say “I’m just sitting here trying to figure out on what planet that’s NOT an invasive personal question.”
I hate people.
You know, situations like these almost make a vasectomy worth the money. 
Or you could just go with: “Well we would have another, but you know that Jonathan Swift really exaggerated about the flavor. It’s just not worth it.”
“Well, we’ve been praying and praying for one; it must be God’s will.”
Bonus points, incidentally, if you can work it into some kind of conversation in which you insinuate that you think that prayer, alone, is how babies are made. Look completely blank or slightly revulsed at any mention of sex, as though you don’t know what it is at all.
If they fall down on their knees and start saying the rosary, you win the internet.
Isn’t that how the Shakers got kids, through prayer? :eek: