The "lines" are personal boundaries a person designates in his/her sexual life. They usually only appear "blurred" to men who don't comprehend the meaning of "no."
Girl: "Hey, I don't think I feel comfortable with that."
Robin Thicke: "Just let me liberate you... I know you want it... I hate these blurred lines."
Gay guy 1: "I don't know if I'm ready for that."
Gay guy 2: "You invite me over, and now you don't want to hook up? Gosh, what's with these blurred lines??"