I was born into a similar situation, my parents were in a casual relationship and in fact in their case they were dumb with birth control. My mum changed her mind on abortion and so I exist.
Thing is I wouldn't want a dad who didn't want me. My dad wanted to be my dad once he realised I was going to exist. He was enthusiastic about taking care of me when I was born and stayed in my life even after my parents permanently split while I was still a baby. I was very fortunate, I also have been able to know my three brothers he had after me who I love.
Not every man can do this, I couldn't if I were a man. Some people genuinely would be miserable as parents and you can't just command yourself to want a life you despise. You don't have to be deliberately cruel for kids to pick up on being unwanted. I know growing up feeling like a ball and chain to my dad would have deeply hurt me and made my self-esteem issues much worse if I felt even my own dad hated his time with me, much more painful than never knowing him in the first place. I know people who never knew their dads and they aren't doomed. I do not think it is right to try and pressure unwilling fathers to go through the motions, it would cause more suffering, it'd be better for him to step back before the kid gets attached if he isn't willing to be a full dad. Life isn't a fairytale and we can't count on everyone becoming superparent through the power of love or whatever.