Maybe you don't assume that, but the system built around parenting does assume that. A clear example of this is "new parent" time off. At my company, new parents can take 4 months off work (paid). So, if you work on a team of a dozen people, and 10 people on the team have children at some point, that means, at a minimum, 2 people are picking up more of the slack than the others.
No, the real issue here is that everyone is paying for this benefit which only helps a subset of the workers. Ordinary PTO—vacations and holidays—is available to everyone to use as they wish. Sick leave is only for those with medical issues, but statistically most employees will end up using it at some point and they all benefit from having that protection since it's (more or less) a random occurrence. But parental leave is not random; it's specifically tied to the employees choices, and some employees are not in a situation where it would ever benefit them. Logically, then, paid parental leave should be an opt-in benefit which employees pay for, not a default part of the general employee compensation package.