NO. For one thing, he is a cartoon sponge, and doesn't have any sexual orientation. Stephen Hillenburg, the creator of the show, has stated that the characters are 'asexual'. (Asexual means that you aren't sexually or physically attracted to anyone. However, people who are asexual CAN fall in love. You do not have to be physically attracted to someone to love them, and many consider such relationships to be more 'pure', as there is no lust or sexual desire involved. Even if SpongeBob was to fall in love, it would not make him straight or gay.)
Although SpongeBob can occasionally exhibit behaviors that some adults, rightly or wrongly, associate with a gay lifestyle, such as holding hands with Patrick, singing, and generally being happy, he is not gay. SpongeBob is simply a fun-loving, forever-kid, and a naive one at that. SpongeBob holds hands with Patrick because he's one of his best friends and he loves him as that, as he should.