It introduces more factors to consider. One, the kids will be involved in the relationship, so that’s more people that have to get along well. Two, they take up time, energy, and resources that could otherwise go to the relationship. Three, the parents are more anchored to their current life, so things like going on vacation or moving become much more complex and expensive. Four, the person will almost certainly have to maintain a relationship with their ex/other parent of the children. Five, if you yourself want to have kids, they may not be willing to do so anymore.

I acknowledge that having kids has its benefits too though. It’s not all bad.