I guess I'm an exception to the norm but I would do what your Bf is doing. It seems both parties agreed to not have members of the opposit sex who are not married allowed in the house over night. I have that same decree in my divorce. It is to prevent someone from having multiple partners (strangers to the kids) over at night so they don't have to wake up and see some random guy/girl in their parents BR. It is to maintain some sort of stability in the lives of the children.
9pm is usually the norm for most kids' bedtime. If he has a problem with it, he can always request the courts for modification. After all, the oldest is 14 yrs of age some would agree 9pm is a little too early to have to leave. Midnight seems more reasonable.
If I were your BF I would probably do the same thing he is. He sounds like he's a good father who is concerned about violating a decree and being held in contempt for it. This, giving NJ a reason to modify < parenting time > and lose the time he slready has with his kids. He's not scared of your ex...he's scared of the judge who may rule in her favor. He's thinking about his kids first...which is a great thing. I'm learning no matter how unreasonable a decree may be or sound, it doesn't matter one bit. If the both parents agree to it then a judge will hold them into account for intentionally violating it.
As for you, you have three options. Leave him if you're tired of the drama, marry him so you don't have to deal with the drama, or support him in either following the decree or modifying it. As a dad, I wouldn't jeopardize my time with my kids for ANYONE, even a GF who may give me some skin from time to time. Blood is much thicker than water. Like the old saying goes, there are more fish in the sea...and that goes for both of you.