Most employers that would accept that of parents would accept it of anyone. Most employers that wouldn't accept employees setting reasonable boundaries or making reasonable requests wouldn't accept them from parents, either. There are bound to be some that only accept boundary-setting from parents, but I assert that it's a very small percent of employers or managers. Maybe I'm wrong.
What I'm proposing here is that most people are too afraid to ask for what they want. Parents have the benefit of having some of those expectations carved out beforehand, but I think that many of them also develop a newfound ability to say, "No, sorry, can't do that."
I have never had any issues getting “dinged” for the fact that I have to stop work at a specific time because meds need to be taken with food. I communicate it clearly to my peers and managers.
In the last couple months I’ve had to miss days to deal with doctors sometimes on zero notice (as in I actually responded to a question from my boss with “uh if this is urgent here’s my cell call in about 30 and I can discuss it but I have to take my other half to the doctor”)
Some of this is being willing to admit it and just simply ask. I didn’t once and tried to work through it and my boss at the time reamed me out for it.
But I’d also never think twice if a coworker said “I gotta take off early, got a date”. Life happens, as long as nobody is outright abusing it who cares?
It sounds like in the situation you described people are abusing it and the managers are just not paying much attention.
For example, someone with kids gets a bigger insurance benefit. Usually managers are pretty forgiving when someone says they need to leave the office at exactly 6pm every day to get to soccer practice, leaving the single people stuck without the same excuse. Oftentimes the single people are asked to do the extra off-hours work "because you have no kids so you don't mind right?"
This is just an extension of that, which is perceived as unfair to the single folks.