You could always try to honestly engage them in that conversation. When they say that you'll change your mind, you could ask them (not with an attitude), "I'm curious about why you think you know what I'll want better than I know that myself. Could you explain why you think I lack that kind of self-awareness on a choice that's that important?"
Or, if they take a more, "Well, you should want to get married and have kids" kind of approach, you could do a similar sort of thing. Ask them why they think that their preferences for your life are more important than your own preferences for your own life. You could say something like, "Okay, I understand that you want me to get married and have kids, and that you think it's both important and enjoyable for people to do that. I get that, and I think that it is true for most people. But I don't think I would like it, and I hope you don't think so little of me that you assume that you know what I want better than I do. It is possible for someone to have a different preference than you do, isn't it? In that case, my question to you is whether you want my choices in life to make you happy at the expense of my own happiness, or if you'd prefer to see me happy, even if that means that I make a couple of choices differently than you did."
I think that if you were going to do something like that, you'd have to do it in a non-retaliatory kind of way. Don't ask in a snippy way, or to get back at them for being pushy, or just to shut them up or piss them off. Ask the same way you'd ask them what they're doing next weekend-- more to get information than to make a point. There's a chance that if you ask someone that way, they might see that they really don't have any reasons. But then again, I'm guessing that there's a significant amount of overlap between people who believe that their own personal preferences are, or should be, universal and the kind of people who don't understand the term "respectfully disagree." So maybe it's a lost cause.
Just as a disclaimer though, I should point out that I almost never had to deal with this kind of thing. My family is pretty respectful of individual choices in these matters. And even on the few issue that they have stronger opinions on, they seemed to figure out quickly that I'm going to do what I want to do, regardless of how they try to influence me.