Recently, former Mixed Martial Arts (MMA) fighter, Josh Thomson, posed a controversial question on his Facebook page, "Should you be allowed to marry whomever you want? Before you answer that, should u be allowed to have more than 1 wife?" He went on to suggest that marriage equality would lead to issues like polygamy, pedophilia, and incest. He isn't the first one to make such claims; many anti-gay activists put forward this ridiculous argument, believing that legalizing gay marriage will lead the way to bestiality and social breakdown. The idea that same-sex marriages will lead to people wanting to marry their pets or cars is just crazy.
Homosexual marriage is no more likely to lead to such issues than heterosexual marriage. There is no logical link between gay marriage and pedophilia or polygamy. Marriage should be a bond between two consenting adults, and it doesn't matter where these two people fall within the sexual spectrum. Child marriage, on the other hand, is usually not based on consent; the power dynamics are completely different than those in an adult relationship. Forcing a pre-teen girl to marry a man many times her age is clearly an infringement of her basic rights. Interestingly, the countries that are likely to have forced child marriages (Pakistan, for example) are also the ones most likely to oppose gay marriage.
Gay marriage doesn't lead to polygamy, either. Countries that have legalized gay marriage haven't seen any rising demands of polygamous marriages. However, there are a lot of countries where polygamy is seen as completely normal, while gay marriage is forbidden. The marriage of three or more people is fundamentally different than the marriage of two people. Same-sex marriages have the same dyadic structure as heterosexual marriages. Therefore, legalizing polygamy is quite different than legalizing gay-marriage. If polygamy were legalized, a small number of high-status, wealthy men would end up with a disproportionate number of women (think of Mughal rulers and their harems). Now this might destabilize the structure of society.
And unless people want to follow certain royal families and end up with genetic disorders and hemophilia, it's best to stay away from incest and inbreeding. Polygamy and incest might create social instability, but gay marriages don't have the same harmful effects. And neither is same-sex marriage a gateway to such matters.
The most ridiculous claim, however, is the one about how gay marriage will lead to bestiality. I shouldn't have to explain this, but, seriously, homosexual marriages are no more a precedent for marrying your dog than heterosexual marriage. Apart from the issue of consent (how can you be sure that your dog wants to marry you?), this argument is completely irrational and not convincing at all. It is simply an attempt to ridicule any argument made in favor of gay marriage, and can be safely dismissed.
Conservatives might claim that gay marriage will lead to social breakdown, but honestly, the only thing that will happen if gay marriage is legalized is that two people who love each other will be able to get married. The world won't end.