Having sex causes siatic nerve back pain why?

Strain/ instability: Certain sexual positions can aggravate an underlying sciatic nerve or lumbar disc problem. If this is new symptom it should be evaluated by a competent clinician. If chronic and documented, then you can pay attention to what positions aggravate and what positions allow you to have pleasure without subsequent pain. If worsens see your physician.

Yes: I see many patients with back pain and sciatica who report increased pain during sex. If your back is sore or inflamed and the nerves are sensitive, the motion of sex may aggravate your symptoms. Having said that, i never tell my patients not to have sex only to experiment with different positions that may not cause pain or to avoid positions that cause pain during the act. This typically improves.