How about making the choices be:
Food preferences:
None
I like to eat meat and fish
No animal products for me (vegan, strict vegetarian)
Other / allergies (contact organisers)
This still means that people with true restrictions, such as
allergies, have to contact the organisers, but we can't reasonably
ask for all combinations in the form anyway (e.g. gluten-free vegan
and lactose-intolerant meat-eater…).