That's not to say your partial understanding is not valid, it's just incomplete.
Not only am I adopted (thus not "wanted"), but neither of my children were planned and my wife and I initially couldn't agree. From my real world experience, and not speculation, it only takes one person to "want" a child, and biology literally has nothing to do with it. I'm also not angry, sad, or pained with regret; it's impossible to generalize how individuals will respond.
All of that said, I'm all for increasing the availability and effectiveness of birth control, and hope that children make their way to an environment of love and nurturing. So, what really can't be understated is the importance of parents and guardians being emotionally equipped to respond to those responsibilities in a way that is positive to all involved.