This article says that it is not.
I partially agree with the premise.
Having a child means taking a step into the unknown. It isn't completely uncharted territority, since others have traveled there before and written visitor guides, but you won't truly know what it's like for your until you get there.
I don't think that means that no part of the decision-making process is rational. We make decisions based upon incomplete information all the time, from traveling to a new destination to undergoing a new medical procedure. We can make decisions to commit to a course of action, knowing that there is no guarantee and that there is some assumption of risk.
While you can't always know with 100% certainty how you or someone else will react to parenthood, there are some ways to have a pretty good guess. My husband always doted on his much-younger sister and was an awesome camp counselor and youth group leader. No big surprise that he's a good dad. My sister's husband always worked with kids with special needs, loved the work and was absolutely loved by everyone. No surprise that he's great with his own kids, including my nephew who is on the autism spectrum. My husband's brother was the favorite uncle with my kids. No big surprise that his kids adore him either.
On the other hand, I would agree that there are feelings that I would have had difficulty describing or understanding before I got pregnant. There are also things that I don't really discuss that much with others, because it would just sound too corny or religious or out-of-character for someone snarky like me. So, before I got pregnant for the first time, I knew that I liked kids in general, that my husband would make a good father, and that we were approaching a time in our lives where kids could fit into the picture. That was the rational part. The part that I didn't discuss is that I sometimes wondered while reading all the pregnancy and parenting magazines is this was all worth it, or that I was slightly terrified of the changes that would happen. I told people about the miscarriage, but most weren't aware of the depth of the grief and the fact that I was wondering if I was losing my mind. It was a mental and emotional landscape that I hadn't known existed. I didn't know that it was possible to physically feel grief, to have an actual sensation of empty arms. I do know that our decision to get pregnant again was very different - it wasn't a "sure, I guess we want kids now", but an actual hunger to be parents. It's a bit hard to explain that to people in real life, especially if they've never experienced anything like that, if you don't want to sound like a lunatic.
It's also hard to explain the joy that we felt upon the birth of our first baby. It was just an instant end to the depression and paranoia, and a moment of pure joy. Again, outside of religious circles, you don't use terms like "pure joy" in real life in rational conversations. So, I'd talk about the mundane and funny stuff, like the first time I had the baby pee AND poo AND puke over me all at once. I had the words for that. I didn't have the words to describe that I didn't really mind that much, because she was part of me and a living miracle and I was still floating on this cloud. I'd complain about the fact that she never slept between 1 am and 5 am, but didn't talk about the feeling of emptiness that I had felt before she was born and how, even though I was insanely exhausted, I needed to feel her in my arms.
It's hard to explain the neat feeling of having your baby grow into a person, and getting to know their own little personality quirks. I didn't know that my sensitive oldest child could move me so much, or that I could be so fascinated with my smart and independent middle child's imaginary world, or that I could learn to cheer on my youngest child's interest in all things sports-related. Rabbi Orlowek once described love as meaning, "if it is important to you, it's important to me". I didn't fully appreciate that until having kids.