TOD
As someone who never married and never had kids or who ever had to be responsible for kids, I have lived my moviegoing life in blissful ignorance of the ratings system.
The only time it would have affected me was when I was still living in Cleveland before turning 21, and the li'l rite of passage I gave myself on that occasion was going to an "adult" film. I hate to even admit what it was, but I THINK the one I went to - and it was really horrible to me, even then - was one of the I AM CURIOUS films (Yellow or Blue, who the hell knows) that was playing in some theater in East Cleveland. Its awfulness did me the favor of automatically wiping itself out of my memory to save me the trouble of doing it.
Anyhoo, I wouldn't know if any films I've seen pushed any boundaries, since I never took it upon myself to be aware of it. At any rate, nothing in particular comes to mind.