'Discrimination' is not "bad" per se. It is merely a social contract to prohibit it to some protected classes. Discrimination literally just means to differentiate, the negative connotations are social and cultural bias, and thus are socially constructed. Society does that because we've found that to lead better outcomes. There's no inherent naturalistic justification to these protections, besides weird Locke-ian attempts to justify "human rights" based on a higher entity (gods). Legislation is drawn as an attempt to resolute differing, conflicting ideas. It is not innate, it is not given by god(s).
Knowing this, then the equivocation of "Irish, Muslisms, old people, women, men, gay people" to "people like to have areas without kids" is ridiculous. The class of a parent and their offspring is not the same as the class of a religion or a sexuality (something that you can't even choose -- whereas you can choose to be a parent (discounting rape, because that is an extraordinary circumstance)).
This is right to the point. Having kids socialize and be integrated in everyday life as much as possible has centuries of proven efficiency, for kids, parents, and society in general.
I don't think there's any solid high minded argument that goes one way or the other, it comes down to the practical repercussions of cutting a specific group from a whole slide social activities.
It really has nothing to do with having or not having R-rated movies. It’s just discrimination against children, and by extension their parents.