The discussion is about a person right too marry whom every they want too. Damn those consenting brothers and sisters for throwing a can of worms into the mix.
The slippery slope fallacy has long been used by everyone trying to outlaw the expansion of civil rights to new groups (Women, black people etc...). Funny thing is, the worst case scenarios never seem to pan out.
But people keep saying "if two men can marry, then why not a man and a dog, or two siblings?"
Well, unlike with homosexuality (of which there is mountains and mountains of evidence that it is an unchangeable, pre-determined trait (as agrees every major psychological association in North America and Europe) there is no evidence that people are born with a sexual orientation that limits them to only being attracted to their family members, or to animals. There are no groups of people campaigning for the right to sleep with animals or their own family. In fact, there has never been any movement to do such. There is not a shred of scientific evidence that such people exist.
So what exactly do you think should happen to gay people? If you say "make them straight" then you must know that is impossible if you do any research into objective science ("Scientists" who work for strongly religious groups are not in any way objective and can "prove" a myriad of things that are false). "Ex-gays" have been proven to be false time and time again, and by the omission of leaders who used to run such organizations (look up Exodus, the original ex-gay group).
So, since they can't change their sexuality, now what? Force them into a life of abstinence? Kill them? Really, what should be done with them?
Has any harm come upon you, or anyone you know, as a result of same-sex attraction? Has the moral foundation of the US crumbled as homosexuality has become more accepted in the last few decades? Is anyone hurt when two people of the same sex love each other? Can anything harmful be blamed specifically on same sex attraction?
Yeah, AIDS is more easily spread through anal sex. But unprotected sex (and other fluid sharing activities) is the real culprit of AIDS transmission. If you blame gay people, then why aren't you campaigning against black people and sub-saharan africa as well? It's not the people, it's the unsafe activity.
Anyways, you're mind likely will not be changed as you have a likely unchangeable belief that being gay is wrong. Same as the people 60 years back who thought it was clearly wrong to integrate society, same as the slave owners who found it ridiculous to allow blacks any access to normal society.
Oh yeah, they were wrong back then, but you're not wrong this time. They're totally different. Yeah, right...