Is it weird that my occupational health questionnaire is asking about my sex life?

Nope. Your sexual habits have the potential to create opportunities for you to be exposed to various diseases. If your occupation involves exposure to blood and other bodily fluids then this information is important to not only your safety but to the safety of those with whom you work.
Unusual . An occ health questionnaire should ask questions focused on health conditions only related to your ability to do your job.