Is it normal that my mum won't let me lock the bathroom door?

Ah, my mother did this for a while. It's a weird way to control you basically; as in there's nothing you can do or go that's out of her reach, and if you do you get berated as punishment. No one likes to hear/read this, but it's emotional abuse to put it simply. Abuse doesn't have to be so dramatic, but just irrational shit like this that can drive a person crazy.

Have a talk with her (it probably wont help, but give it a try anyway just for the sake of trying). Tell her that you're a growing young man, and you need some privacy, especially in the bathroom. If she gets upset, lock the bathroom door anyway, even put a sign saying, "Mom, respect my privacy please," on the outside every time you're using it. It'll make her mad, and you'll get yelled at a lot, but you're standing up to her craziness; you're growing up and she needs to learn that.

she wants your cock

Go into the bathroom, lock the door, then escape out the window and get lost for a couple of hours.

Yeah, because she doesn't want you JACKING OFF.

There is something wrong with your mother.

That ain't right.

Even if there’s no sexual gratification involved, it’s still very weird and controlling of her.

OK so when she comes in look at her and stroke it. You'll know quickly what she's thinking

How many kids u got if none then she did her job ßut that bathroom shit not normal

Bucket of hydrochloric acid on the top of the door.

Sorry guy. You have not told about other family members. Is your mom's attitude same with others, including your father? If you are the only one child, there must be some horrible psycho problem with her.
I am not inclined to agree that she wants to control your masturbation, as it can be done any time (when you are in bed or she is not at home).
Try to understand her past. There must be some unpleasant experience. My sister's husband got heart attack when he was taking bath, the door was locked. No one at home could judge what had happened behind closed door. When it was too late, knocking at door proved useless, the door had to be broken. He was dead. Now nobody locks the bathroom door.

Or could be you are on drugs and she knows it and doesnt want you doing it in HER house.

Sorry but you want privacy. You're a man now, go get your own place. Until then mom owns you.

Looks like it's going to be a long two months with her pounding on the door and getting mad.

She knows what people are doing behind locked doors...