I have no problem with same-sex relations/marriage, but...
Well, if we are honest with ourselves here, the correct answer is "Nobody knows". Why is this the correct answer? Well, almost every legit religion on the planet seems to be extremely adamant about same-sex relations and they are very strictly against it. Although one may state that religion is not real, theologians and historians have been proving validity of the information contained within the books as well as the connections between all texts.
Let us assume there either is some divine being(s) or some other extremely knowledgeable source somewhere in the universe that approached our ancestors in the past, passing down knowledge to us that we now call religion. Until we know what they know or meet them face to face, we have no way of knowing if it does harm us in some way. Face it, for all we know, these "Gods" could be humans from the year 2,200; a time where same-sex relations somehow led to the ultimate destruction of mankind. Maybe they couldn't come back to this time and tell us because we would look at them like nut cases and throw them in a padded room. So they instead traveled back to ancient times and started us on the path we exist in today. Can you say for a fact this is not true? No, so we truly don't know if it is harmful.
I find it interesting how people are annoyed by homosexuality when it has little to no impact on their personal lives. Though there is some impact that could happen with people that have children who are against homosexuals, though my retaliation to this argument is that it is just another lifestyle and just adds variety to cultures and makes people grow up to see homosexuals as a normal thing.
Here is a question for those who oppose gay marriage: A same-sex couple whose lives have no effect on you get married. How does that harm you? There are more harms in denying gay couples the right to marry than there are harms in homosexuals marrying; gays and lesbians are being treated as second-class citizens.