But I'm worried that if I plan not to have children, I'll change my mind later.
Why is this a problem? Freedom of choice is a good thing to have. If you really want to commit future-you to the choices that now-you wants to make, there is a variety of surgical procedures that are moderately hard to impossible to reverse. But again, why do you want to give up future choice?
How will I know when my preferences are stable enough to plan around?
Future is uncertain. Your preferences will never be cast in stone and that's a good thing. Whatever you plans are, they should take into account the fact that things will (not may) change.
Freedom of choice is a good thing to have. If you really want to commit future-you to the choices that now-you wants to make, there is a variety of surgical procedures that are moderately hard to impossible to reverse. But again, why do you want to give up future choice?
I'm worried about getting a woman pregnant, and I'm not sure reversible birth control is effective enough.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.