I too feel the secondary lead makes a somewhat creepy boyfriend. Not a terrible boyfriend, but definitely on the controlling side.
But it's utterly clear that, bottom line, she just isn't that into him compared to how into the lead she is. Her hormones respond to the secondary guy at a sort of sparkler level, just generic "He's a handsome guy", to the male lead at full fireworks show level. No matter what a moron he is, this is not ultimately something that can be overcome.