Good points. I think we are supposed to identify more with Lee. We probably aren't supposed to think Patrick is horrible, though. When he finally sleeps with the second girl, we see him grinning, it's part of a "things are getting better" montage, and I genuinely think it is supposed to be cute. A "boys will be boys" kind of thing that is perhaps intended as a regionalism. But it's actually a weird discordant note that doesn't work.
And it strains credulity that the girls don't know about each other. They are even at the funeral together, where Patrick is sitting with girlfriend #1. How did he introduce them to each other? "This is my girlfr... I mean friend..."
The issue is not that he's having sex with multiple girls. Fine if that's your thing and they both know about it. The problem is that to carry this out in secret he has to operate as a pathological liar and is constantly betraying both of them.
He's in a long-term relationship with girl #1. She goes out of her way to be emotionally supportive and even protective of him, so she clearly believes it's serious. How can that not end in catastrophic heartbreak? Can't even attribute Patrick's behavior to coping with his dad's death, because he had been pursuing girl #2 before that.
I might even tell myself to lighten up, but this is a realist film in which we are definitely meant to take the characters seriously, internalizing their relationships and behavior. I would be interested in an interpretation that responds to my complaints.