I've shredded the inside of my upper and lower lips from covering my teeth during super-long BJ sessions and had my thighs bruised pretty badly from one night of rough sex with a super-skinny guy with these jutting hip bones.
The worst cramp I've ever gotten was one in my hip while in missionary position; I basically started shouting, "CRAMP! CRAMP!" and shoved him off of me. Once my toe cramped up, and we had to stop because it was so uncomfortable I couldn't focus on the pleasure
but once it was sorted out we got back to business.
I find yoga poses meant to stretch the hip flexors are best for preventing sex-related injuries.