Let's start by saying, you can't take the title as an accurate representation of this movie. It suits it very well though, just not in a literal sense. Some people didn't like it because they expected an original comedy about open relationships and got a romantic comedy about two emotionally messed up individuals. And if you aren't expecting it to be something else, this movie absolutely nails it.
I know not everyone wants to see a movie about those kind of characters, but I'm really sick of rom-coms about people who have a healthy relationship with sex unless they're a bad guy or someone whose not mean to find love. I thought it was really refreshing to see characters with these issues and the movie was well done. Sure it's a romantic comedy with all the right clichés and cheeky music, and sure the characters aren't relatable to everyone, but for me, it was just perfect.