The kid still has a father.
Even if they meet and marry other people, the baby still has two parents who hopefully will be mature enough to maintain a respectful relationship with each other and a loving and involved one with their offspring.
In my experience (I’m old now—I’ve seen a lot! LOL) it is fairly rare for that to happen. Usually one parent or both dislikes the other and things do not go smoothly. Many people tend to feel no qualms about using a child to get back at the other parent. And without fail, they ALL feel justified. It’s rarely a good situation.