I guess it's time to throw in my two cents.
Let me start of first by saying that I am a non-religious person. That is completely my choice, and with that, I am coming into this discussion with an unbiased and accepting mind.
I will also make it clear that I am not "for" homosexuality, nor "against" it. To be "for" homosexuality would be like being "for" people with brown hair, or being "for" people with a certain accent. I don't see the point in making a deal about supporting something that is completely natural and should not be discriminated against. But that is unfortunately not the case with our world, and the idea of homosexuality is viewed as an abnormality among our human race and many are too afraid to accept it as a natural occurrence in the human genetic makeup.
From a scientific point of view, there is nothing "wrong" with being homosexual. It is just that the sexual attraction to a particular gender has been given to someone of the same gender. This of course is not the only case, and to say that "homosexuality is not a choice" is not always true. There are some people who may choose to experiment with the same gender, but that is still a choice of theirs and should be viewed no differently to someone who chooses to wear a blue tie instead of a green one for a change. But, in most cases, homosexuality is not a choice, and is just something that they are born with. Religious beliefs aside, this is a genetic fact.
All humans are not the same. And yes, one man and one woman can make a child, but that does not mean that makes it any more normal than two human beings who cannot have such a privilege of having a child. All humans are not the same, and just because a man/woman couple is able to have a child, doesn't necessarily make them ore deserving or worthy to raise a child then same sex partners.
As a non-religious person, I also view marriage as a foreign concept. Like religion, it is something that humans have created to give our lives some purpose or meaning; something to live for and live towards. But in this day and age, with divorce rates constantly rising, it makes one wonder; why should so many untrue and unsuccessful marriages be blessed to opposing gender couple, and not given the same chance to homosexual couples? I'm not saying that all heterosexual marriages end in divorce, and I'm not saying that all homosexual marriages would not, but it should at least be able to happen. In a nutshell, it really is an abuse of human rights, allowing certain people of a genetic orientation to marry, and others of a certain genetic orientation to not. Imagine the outcry if those with downs syndrome were not allowed to marry? Those with blonde hair, or those with brown skin? Nobody would even consider not allowing these people to get married, yet there is such an objection to two people of the same gender sharing in such an event?
But marriage aside, a piece of paper cannot stop what is a natural occurrence. The main issue is that some people seem to only believe in "straight" (normal) people and "gay" (abnormal) people. When in reality, there are many types of genetic orientations, such as bisexuality, pansexuality and asexuality, just to name a few. The most common and most misunderstood would of course have to be bisexuality. And, like homosexuality, for most it is not a choice. It is genetically possible to be attracted to both men and women. And just because a person is attracted to both does not mean that they will eventually end up favouring the opposite sex in hopes of setting down and finding a family, if they fall in love with somebody of the same gender, that is perfectly normal for a bisexual person. The problem that lies here is that some homosexual people use bisexuality as a safer way or "stepping stone" to coming out from a perceived heterosexual to their true homosexual self. But just because this may be abused by some does not make it a faux sexuality in the slightest.
Pansexuality is similar to bisexuality, in the sense that an attracted to both genders may be present, but the gender is not a factor in the attraction. Pansexuals see all humans as individuals, regardless of gender, and may love whoever they choose to. Bisexuals on the other hand do recognise and accept a difference in gender, but are naturally attracted to both. Asexuals are not attracted to either gender, or have no genetic sexual identity or preference.
This brings be back to the bible. As a non-religious person, I have not read the bible, but I understand that in at least some areas it is understood that homosexuality is a sin. Much like all religions, and not to pay any disrespect to the religion, but the bible is old. Times have changed. Just because something was viewed as immoral or unjust then does not mean that it will stand the test of time and remain immoral or unjust for an infinite amount of years to come. It is only natural for us as a human race to expand, to learn and to grow. In recent years homosexuality has definitely become something that is no longer viewed as a terrible sin by many, but there are still many that do view it as just that. I ask you all to keep this in mind; think about just how many thing in our life today that we have that were not mentioned in the bible. Then hypothesise what our world might be like today if such things were deemed sins in texts such as the bible. From the computers that we use to the coffee that we drink to the music that we listen too... imagine if this was viewed as sinful in our religion? How bad you would be made feel if every time to took a sip of coffee you couldn't help but feel someone staring at you, judging you.
Just some food for thought. But for now, I will leave you with this video:
https://www.youtube.com/watch?v=XM2J7nOp3nU