aggressive

I have a male German Shepard that a bottom for and he’s normally super nice but during play he gets really aggressive like growling and barking even biting me I’m not doing anything that can cause him to act this way I assume the position and sometimes it hurts so I move to help guide him in or...

I’ve been looking into getting dicked down by a German shepherd or husky and I found someone willing to do it but the dog is quite aggressive when it comes to mating. I tried once and he pushed in deep and it hurt really bad he then bit the back of my neck. How should I approach next time?