There's a difference between reasoned positions and rationalizations. Most of has have both in our mental baggage, myself included.
A reasoned position is one that you take after looking at the evidence and bowing to what the evidence tells you.When you decide on a course of action based on a reasoned position, you combine the knowledge of what the likely consequences of that action are with your own fundamental principles telling you which outcomes you'd prefer to have happen.
A rationalization isn't precisely the opposite of a reasoned position, though it's often presented as such. A rationalization happens when you pick your course of action, and then look at the evidence selectively to find things that tell you that this course of action is in accord with your fundamental principles.
These two modes of thinking can quite often lead one to the same conclusion, but rationalizations suffer from a serious risk that your actions will lead to a world that isn't as closely in accord with your fundamental principles than you would like. On the other hand, establishing reasoned positions is exhausting, requiring constant re-examination of your chosen courses of action, and the occasional devastating moment when you realize that you've been on the wrong path for some time, doing more harm than good, and needing to reverse course.
The saying "you can't reason someone out of a position they didn't reason themselves into" is about this divide between reasoning and rationalization.
I try to limit my rationalizations, even if I'm occasionally guilty of them. I try instead to keep my actions and principles in proper alignment, and often challenge others to do the same. One of the biggest rationalizations I've still got rattling around in my psyche is this one: I didn't reason my way into the position that I shouldn't have sex with kids. I adopted that conclusion then came up with rationalizations for why that is the correct course of action.
The evidence tells me that if I want to normalize adult-child sexual relationships, the most effective way to do so is for as many people as possible to engage in them. Stygma is removed by familiarity, and the world as a whole benefits from the progress made. If those of us who care, and who want compassionate, consensual, loving relationships weren't the ones avoiding relationships, we wouldn't have seceded the field to those individuals who don't care about love, consent, and compassion.
But to follow this course of action requires a sacrifice, and it isn't one I'd be making alone. My hypothetical lover would be put through the wringer along with me, and the idea of asking someone I care about to do that causes a nearly instinctual recoil.
But to shine the harsh light of reason on my rationalization once again, I hold as a principle that competent human beings have a right to make their own decisions, and the evidence tells me that young people are quite competent to do so. They certainly would for anyone I would consider entering into a consensual physical relationship with. So do not my principles demand that I let them agree or not based on their own will, rather than attempt to shield them from such a relationship out of a misguided protective impulse?
I don't have all the answers. I'm still struggling to put words to some of the questions. I know there's something wrong with my thinking on this one, and until I've got it sorted out, I'd rather not act. But isn't that just one more rationalization for what I was going to do anyway?