Paid time off for whatever reason is a benefit you and the company negotiate for. If the company decides to grant parents time off that non-parents don't get that's fine. That's just the company's opening position when they're negotiating with people who are parents, in much the same way that people who drive to the office might negotiate for a parking space or people who have a long commute might negotiate more hours working from home.
What you get in return for your hours is basically nothing to do with other people, and what they get is nothing to do with you.