How can I convince my parents to let me have snails instead of a dog?

Your parents are somewhat right about the rapid multiplication rate of snails but if you really like them then I don't see any harm in them allowing you to have at least ONE snail, good grief lol. And just because you may prefer snails to dogs doesn't mean that "something must be wrong with you" as that seems to be what your father is implying. It just means you're different, which isn't necessarily a bad thing :)