I don't think anyone is accusing people of being a slut or shaming people, not on this thread anyway.
What happens when one father is able to pay their child's way through university, pay for driving lessons, pay for the big white wedding and help out with a house deposit and the other father refuses to pay a penny? Or ones father wants to take his dc on holiday to Disney world, and the other father won't even see the child during the holidays? I could think of so many scenarios where children living in the same home are treated completely differently by their NRPs, and while it might turn out ok for some children, it could turn out horribly for some others.
There are just so many potential problems that could arise. If people are going to have children by more than one person, then they have a responsibility to their children's emotional welfare to think things through from their existing children's and their potential children's points of view.