I love this movie and find it completely unfair to say that this is all about sex -- the fact that the other reviewer made a point of emphasizing that the characters and skin flick in question are gay makes me more than a little suspicious of the reviewer's biases. It's a relationship movie. Yes, there's sex involved. That's because romantic relationships usually involve sex. It's not hard to see past it to the other emotional content.
This movie is a beautiful story about a family that finds themselves in a social crisis at a difficult point in their lives. The kids are played realistically with a well-observed combination of independence and neediness that feels really authentic. The moms are just as good - really touching in spots and hilarious at other times; both have weaknesses that the other tries to work through and the conflict between them feels really true to life.
The movie's primarily about transitions - in the moms' relationship, and in the kids' lives (college, sperm dad, etc.). Mark Ruffalo's character comes off as a real tool and I, at least, have definitely met people like him and enjoyed seeing the way his interaction with the family plays out. There are a lot of short snippets that give really great insight into the characters without having to explain too much. It's set in California and I loved the moments where there are peeks of the characters' super-granola side (in other words, the times when they're so typically west coast liberal it's funny).
I can see a lot of different people digging this movie - older teens can relate from the kids' perspective and parents would recognize a lot of the difficult and moving moments. Overall, highs and lows, a good rhythm, good acting, well written - if you like dramas, are in your twenties or older and want a good quiet date movie, or are older and into something that'll tweak someheart strings, this is a good choice.