I don't get the opposition to Gay Marriage?

I just don't get why people oppose it? I'm Catholic and, see no problem with it. Other than spouting religious quotations, what is legally wrong with it, I mean after all it is a legal issue, right? It makes no sense to me? People tell me it's inevitable but, there are some serious people hellbent on ensuring such a thing never happens. Oh and, do people actually and, foolishly think that legalizing such a thing will lead to Polygamy, Pedophilia marriage and, Bestiality marriage?