My official opinion is this; Gay marriage should be legal and it won't harm normal marriages. Whenever somebody says it will harm marriage, they're usually talking about, "Bibical marriage." Marriage that says it can only be between one man and one woman. Trying to push that into law is basically trying to enforce a religious belief down on everybody in the Country through the law. Which is itself, somewhat illegal itself according to the 1st amendment of the Constitution. Quote, "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof." To enforce a rule based on a group or somebody's own personal religious beliefs, is intruding on all those who religious beliefs say that the subject in question is ok to have. Not all Christians interpret the Bible as saying that homosexuality is a sin, nor do all religions. I mean, if the majority of America's population were Islamist (I'm just using a example here, I have no problems with Islam), it wouldn't be right for them to enforce a law in the country that all within it cannot eat bacon, when many of those within the country says eating bacon is ok because their religion has no problem with it.
I personally think marriage should be up to the Churches and other religious places of worship, and that government's only role is to give out the Marriage licenses. If one is against gay marriage, they should just be a member of a church that doesn't recognize gay marriage or provides gay marriage services. This way, those who want to gay marry or support it can be part of a church they know of that does support it, while those that disagree can be part of a chruch that doesn't. Some of course, would still have a problem with gay marriage. Some will of course, point to the Bible talking about Sodom and Gomorrah, saying that if Gay marriage is allowed, it will offend God, and God will personally see to the destruction of any person, church, home, town, city, state, or country that allows Gay marriage. That they're trying to protect them from offending God and receiving his wrath, including any innocents that may get harmed someway in the crossfire. Again, this is enforcing their own personal religious beliefs through law on those who may not share the belief.
Another personal belief I have, and this is just a theory I personally have, is that it maybe somewhat a genetic defense of sorts. We as a species of human beings, at least some of us, strive to see that our DNA and it's history passes on to a new generation. That those who fear gay marriage, fear their children would view it as saying they can find "love" within those of the same gender as themselves. And since two people of the same gender can't really, "make a baby," the parent thinks to themselves, "Oh no. My child is gay. My family line ends with him or her, because he or she can't make babies with somebody the same gender as he or she." Yes, indeed, Gay people can go to sperm banks and so on to make a child, but perhaps this information doesn't register in the minds of some of the parents. That it's not human, and therefore, evil. They want to see that their family line continues, and want to see their child marry somebody of the opposite gender so they can possibly have children of their own naturally. Again, this is a bogus thing for those against gay marriage to act.
Again, I say, it should be left up to the churches, and government's only role in marriage is to give out the Marriage licenses. And yes, this means I'm ok with polgamy being practiced in every state if one's own religious beliefs says it's alright. Whom am I to say it's wrong, unless if it's trully something that can harm somebody, like stealing or killing someone. That's just plain common sense, not religious belief. The only things I don't think should or would be allowed, is those bellow 16 or 18 depending on the state cannot get married unless if approved by parent or guardian, and no animal marriage. Animal marriage is just a stupid idea and something people pull out of their asses to say where legalizing a certain kind of marriage will lead us. Again, that's just stupid BS.
In conclusion, Gay marriage is ok and it shouldn't harm anyone's marriage. Period.