A wily trial lawyer I once worked for delighted in asking outrageous personal questions, and people almost always answered him. "Never forget," he told me, "a question asked is not necessarily a question you have to answer." That got me thinking. When I was feeling tender, defensive, or vulnerable, I wasn't ready to explain my situation. Then I decided I had every right to skirt the subject, because my reproductive status was no one's business but my own.
If it's your job to figure out when to share this, it's also a partner's job to figure out when to ask. Wanting kids isn't like monogamy or working for a living, where until instructed otherwise people can basically assume that's your plan. More people than ever are deciding they don't want kids. The fact that you're one of them is not shocking, confronting or even especially unusual.