Why can't I say "我的腰不好”
腰 or 腰子 is slang for kidney. In traditional Chinese medicine theory, kidney is responsible for producing and storing sperms. Thus 腰不好 becomes a slang implying weakness in bed.

It takes a cheeky friend to twist 腰不好 towards sex; In a lot of situations saying 腰不好 is fine. If you are unsure, it is always OK to say 腰疼 or 腰痛.