What's wrong with refusing to eat one type of meat? I don't eat fish for the same reasons.
Same thing with shellfish (also forbidden by Kosher rules). Not only do they tend to collect nasty bugs from any human sewage in the water, but they have some truly scary toxins in their own right.
None of those are an issue with modern farming and aquaculture, though.
What probably happened was that a whole village got wiped out with (say) Paralytic Shellfish Poisoning, and the priest from the next village decided that God really, really didn't want people eating oysters any more.
Why don't you eat fish? I can't work it out :)