I think SpongeBob is way too childish to have a relationship. I don't think he's got special feelings for anyone. And I doubt it would ever happen that he'll have a girlfriend, because some people (like me) doesn't see Sandy (for example) and SpongeBob romantically involved, so they would be shocked if they suddenly get together. And then again, books about SpongeBob and Sandy's "love" is just fan fictions that someone wrote and send it to Nickelodeon, who then started to sell the books. And for those signs in the show, that SpongeBob likes Sandy, I guess it's all friendship, because SpongeBob likes Squidward and Patrick a lot too (I do not say he's gay but he do like them), he likes everyone. And the creator really said that all characters in SpongeBob SquarePants are somewhat asexual, so they aren't gay or straight.
This is my opinion.