I don't know if I agree that you can step in (excuse the pun) and say that you're going to be someone else's kid's father if the kid already has a father. You hear these stories all the time where a kid isn't discouraged from calling a new partner daddy and that's not really right. Kids being used as weapons is wrong and just because a relationship doesn't work out that does not reduce a father of a child to a mere sperm donor. Even if the pregnancy is unplanned or the product of a one night stand fathers do have rights and responsibilities.
fathers who handle responsibility have rights
sperm donors take no responsibility and have none
but the man who RAISES a child, makes the sacrifices and takes the responsibility is the rightful father(in absence of a bio who has taken the responsibility instead)