Pain in left ovary during sex. Is this bad?

Complex topic. Pelvic pain is complex as there are several organs in the pelvis. The ovaries, fallopian tubes, uterus, cervix, bladder, appendix, intestines, colon and rectum can all be the source. Pelvic pain requires and examination to help organize the evaluation.
Not necessarily. I doubt the pain is in the ovary, though. It may be in the tube or even uterus and it's most likely a cramp of some kind. I would recommend referring to your primary doc or an obgyn for full history and physical exam.
Can be normal. It depends on how often this happened and which positions caused it.