I wish that makers of videos presenting “conservative” views would use less Wagnerian music. With that caveat, this video from the Family Research Council makes a good argument against the legalisation of same-sex marriage from a Protestant point of view. It relies far more upon Scripture than on natural law, but for practicing Christians it will make a lot of sense. The interview with a Massachusetts man who was jailed because he objected to his kids being taught about gay marriage is pretty disturbing.
However, what is needed is a viral video which makes crisp, robust arguments based on common sense. C-38: The Movie, a Canadian documentary, did a great job, although it was quite long.