Well I guess I can give a basic rundown of some of the arguments and their validity: not necessarily saying I agree with all of them, but they are some of the ones that do have good backing.
From a religious perspective, it's pretty straightforward: God says no. Many people say this is stupid, but it actually makes perfect sense. If you believe in a God that created the entire universe, this God obviously also set everything, including the rules, into motion. Thus, it logically follows that the God who created the world also has authority over how things should go on it, much like you set the rules for a game that you invent, or a model you create; as the creator, you have a perfectly legitimate authority over your creation. Thus, from a religious perspective, it makes sense to oppose homosexuality if God says it's wrong. But contrary to popular belief, people of this persuasion rarely actually hate gays: Westboro Baptist Church is nothing but a loud, irritating minority. And there is plenty of argument within religious circles over whether morality according to that religion is enforceable in a free country (which leads in to the sociological issues).
From an evolutionary perspective, it makes little to no sense; at the very least, the existence of a "gay gene" (meaning some sort of biological reason why a person is gay; no idea why the word "gene" is part of the term) is completely unfounded. Evolution progresses when a small mutation happens, and if that mutation is beneficial and helps the creature survive, it will, and it will mate and spread its mutation. So let's say that, somewhere back in the history of evolution, an animal received the "gay gene." It would be incapable of reproduction. Animals, acting on instinct based on their biology, would not have the sense to know, "If I want evolution to continue, I should reproduce;" they would simply do what they are biologically programmed to do. If they are programmed to have sex with the same gender, they would only be rejected by the rest of the pack unless another animal of the same gender had the same mutation (which, considering the already astronomical odds against these "beneficial mutations" in the first place, is more than unlikely); but even if they do find a partner, the "gay gene" would not spread. Beyond that, contrary to popular belief, homosexuality cannot be caused by genetics alone; it is dependent on many social and environmental concerns. Even the studies regarding biological homosexuality admit this. The point to all this is that there are many good reasons why any sort of biological need to be gay can be doubted, and even denied outright. Again, this is not all the information, so you can come to a different conclusion, but this is more than a valid argument.
Sociologically, we have the family unit to consider. It is more important than you might think: the family is the building block of society. It is how children are raised, how they are cared for, how they are introduced to a society, culture, and world that they will have to spend the rest of their life in. It is vitally important to the continuation of a civilized world. I am not suggesting that social and cultural acceptance of homosexuality will outright destroy the concept of the family unit, but there are many effects, both good and bad, that it could have. It is rather foolhardy to rush into it, especially considering the lack of certainty regarding its validity as a natural part of life. Beyond that, for the moment at least, it is still a very debated topic. If socially and culturally accepted, homosexuality will be taught in schools no differently than heterosexuality. For the good amount of people that still oppose the idea, their children will be taught against their will on a daily basis. Point being, this issue is at a point in our society right now where the full rights of one group means trespassing on the rights of another. It's not an easy issue to deal with.
I'm not saying you need to agree with any of these, but there is validity to them. They are valid concerns, not irrational fear and/or hatred. I've stated my opinion, but I would like to say again that my opposition to homosexuality does not mean I have anything against homosexuals themselves. For one reason or another, the issue of homosexuality is the only one in which we tend to equate disapproval of one's lifestyle with hatred of said person: this is rarely true on any other issue, and it's rarely true on this one. That alone means there is a big difference between "homophobia" and opposition to the homosexual lifestyle.
. No but seriously, I have friends who are severely homophobic and it's for a couple reasons.
