So, my little story on that subject:
A friend of mine is Muslim and at the central railway station of my city (which is a big place) we somehow got into that topic - he believes that homosexuality is a sin. So we're standing there at the railway station, talking hours and hours about why it should be a sin or not, or why these people are either sick or made a personal choice in their lives.
Now that you have that image, I'll share some parts and thoughts from that discussion. I'm a person who believes in the freedom of people. I'm also a person who doesn't believe in god anymore, but was raised as a Catholic. So in my opinion, all rules we set for our society were created by us - there is no absolute right or wrong, it's all a matter of perspective. With this, there can't be an absoulute "sin" - a sin is only what we want it to be.
Think about incest for example, why is it forbidden, why do we go all "eww" when we think about that topic? Incest was pretty normal in older ages, for example in the Middle Ages. But we as a society saw at some point that incest leads to new born children which mostly are handicapped, so we created a rule/sin against it. (Not supporting incest or anything, but just thinking rational about it.)
Also, according to my friend to whom I was talking, being homosexual could be a mental disease. He brought up that if you are for example suffering from depression, we say that this is a mental disease which should be cured. And that's right - you could define everything which let's people drift from the norm, in my case e.g. being a brony, as a mental disease. We as a society and we as individuals can define what freedom or personal life choices are and what mental diseases are.
So it all really comes down to wheather you believe that being homosexual is an "absoule wrong" thing to be or not, which according to my friend, in his religion is in fact a sin also in order to somehow protect society. Or wheather you believe that being homosexual is not a personal choice, but a disease, somehow mentally triggered and should be cured. In my opinion, being homosexual is absolutely a matter of freedom and personal choice, and I don't even have the right to tell anyone otherwise. That leads to the conclusion, that homosexuals should in fact have the same rights as anyone else on this planet.
But I also respect people who say otherwise, IF they just think that this is a sin/disease/not good for our society and don't try to actually go against those people and steal their rights. After this discussion, me and my friend shook our hands and we still like each other, regardless our thoughts on this topic