It sounds like they're not really involved. Chances are their kids have watched plenty of movies with sexual content and they just don't know about it.
If i see a "teen" rating, I would assume it has the same guidelines as PG13. I don't care what the other thing says, because let's be honest here, if you're too lazy to look at the app itself, you're too lazy to look up and memorize the exact guidelines.
Then when I consider what is allowed at PG movies (Uhh Barbarella for one pops into mind) and what is allowed for PG13, I feel like most parents should know one of 2 things- either that ratings can't 100% be trusted, or this is to be expected for TEEN rating.
So if you as a parent really care about this topic that much, then it's on you to monitor it.