Set clear boundaries, and stick to them
What you need is the Bean Dip Response. This is a boundary-setting technique described by a therapist named Joanne Ketch. Somewhat ironically, it was initially developed as a response to unwanted questions and advice about parenting and is now somewhat famous in some online parenting circles. However, it can easily be adapted for intrusive behavior about any life choices.
Essentially, this is a technique for setting personal boundaries when you don't want to engage with others about a personal decision that you have made, like whether or not to have children. Some highlights from the article (bolding mine; anything not in quotation formatting is my interpretation):
The best thing is to assert your boundary and not try to defend your choice.
So how does that work?
Deflect, deflect, deflect
Just state your decision and move on: When people ask "when are you going to have kids?" you say "That's not in our plans right now. Have you tried the bean dip?" (Of course, you can substitute "How about them Dodgers" or "Isn't this weather crazy?" or any other innocuous redirection.)
If they ask you why you don't want children, you say "it doesn't fit in with my/our life plans. Let me get you some bean dip."
When they say "You'll change your mind when you get older" you respond "This is working for us right now. You know, the bean dip is especially great with these artisanal crackers I found."
"Are you sure you don't want to keep your options open?" Answer: "Yes, thanks! Have some bean dip!"
"Don't you like kids?" "More bean dip?"
And so forth.
Be firm, be kind, be consistent
Now, with some people . . . [t]he offer of bean dip will not be sufficient to redirect them. They are either not intuitive to gentle redirection or they have emotion tied to the issue and a desire to “go there” more deeply. . . . You may be able to anticipate persons for whom this is true: If it's a pattern of intrusion, for example, seen in other circumstances. In these cases, the redirect will need to be backed up with action (like hanging up, leaving the room or even the event, unfriending them). Remember, boundaries are not about forcing another person to comply. You cannot “do” that. Boundaries are about what YOU will do/not do.
Exactly what this response looks like will vary, depending on your relationship with the person. If it's mainly a social media friend, simply ignoring or blocking them may be the best option.
For your parents or hers, you will need to take a different approach (remember, the urge to grandparent can be as strong as the urge to procreate, but with less control over the outcome). This really needs to be handled by the respective children of the buttinskis (in other words, you talk to your parents, she to hers).
You may need to get really real with family members who won't let this go. Talk to them in person, if possible. Look them in the eye, and say something like "I know you love me, and want the best for me. I want us to have a good relationship. This is a decision I feel good about. When you keep bringing this up and criticizing my life choices, it makes me not want to spend time with you. I'm not going to discuss it with you anymore, so let's focus on the good things that are already in our lives."
After that, you will need to be really strong about actually disengaging if they do bring it up again: "Mom, we've talked about this; you know how I feel. I've gotta go now. Love you, bye." Do it kindly and consistently and eventually the message will sink in.
Don't explain if you don't want to engage
A key element of this technique:
[D]on't confuse setting boundaries with trying to convince someone of the rightness of your choices. . . . Don't defend your choices beyond generalities, and then only once or twice.
Finally, an important corollary to the “Bean Dip Response” is reciprocity. . . . [I]f you post [your opinions about the subject] on Facebook (or communicate them in other ways), you invite (and therefore solicit) feedback and advice. You need to give the “other side” the appropriate respect in the same manner you’d like the respect.
Which is to say, don't ever tell parents that you "don't like kids" or "dislike most kids"! They will likely feel this as a personal attack on their choice to have children or, worse, on their kids. People with grown children, those who hope to have children in the future or who wanted to have them and couldn't, and even those who are very attached to other people's children might have a similar defensive reaction.
Additionally, if you try to explain your decision on a rational basis your interlocutor may take that as encouragement that they can change your mind if only they come up with a good enough argument. If you present the decision as a personal choice, full stop, you don't give them anything to hang their argument on.
If you must explain, aim for vague, neutral language (and immediately follow up with some redirection, so they can't try further to nail you down): something like "It's just not a priority for me. Bean dip?"
Please note that this is much harder than it sounds. I have employed this technique in various situations, and there is always a temptation to defend your choice or try a humorous/sarcastic comeback. It takes practice to be able to stay consistent with simply stating your decision, sans justifications, and then changing the subject. It's a good idea to practice some neutral responses and plan some ideas for what you're going to put in place of the bean dip, especially when you know you're going to be in a situation where the subject might come up.