Bollykecks there is a world of options between casually dating and planning more children with someone!
As a lot of (I think most) people are saying, it depends on the circumstances. Where children have been living in a family unit with both parents and the parents relationship breaks down, the top priority of both parents needs to be the quality of their existing children's lives - first and foremost their emotional security and self esteem and confidence and relationship with both parents, but also their material well being.
Just as you only have a third child who will be a full sibling to your first children within an existing family if you feel sure that aside from perhaps in the few days around the birth your older children will not feel replaced and will not suffer a reduced quality of life or relationship with either parent, so should a parent prioritise their existing children's quality of life above as yet theoretical unconceived children in a new relationship.
Children who exist trump children who don't exist, essentially.
It's easier to minimise impact on existing children in a simple family set up where children live with both their parents obviously. As soon as parents set up seperate households they have to work hard at ensuring that their children's lives and foundational primary relationships are not of inferior quality for the break up.
Add step parents to the mix and of course it becomes even harder, and start a second family and it becomes harder still to ensure the first children don't suffer for their parents' wants.
Blended families can work, but it's obviously by no means automatic and the default is that the first children are relegated on multiple levels to second priority. Where time or money are in short supply or the second batch of children come along too soon after the original family breaks up, or one parent moves geographically a long way away, prioritising living with a new partner over their existing children, this is inevitable.
Sometimes it can all work well, but the circumstances and passage of time and finances and relationships all have to align for that to happen.