From a scientific standpoint, most scientists believe SOME are born gay, SOME aren't gay and are unhealthily unfluenced to choose to be gay, and SOME choose.
In my opinion, although I'm ignorant on the science that says people are born gay, for the most part I think it's backed up by research. I am not ignorant though on unhealthy individuals, society's influence, and their personal choices to be things they are not. In my opinion, some people are born gay and are gay, some are born straight and because of unhealthy environmental factors are gay, some are born straight and choose to be gay for unhealthy reasons, some are born gay and choose to be straight for unhealthy reasons, some are born gay and because of unhealthy environmental factors are straight.
To say that all gay people are born gay is a lie. Some are confused of their sexuality. Some are straight and because they are unhealthy are gay. Some are gay and because they are unhealthy are straight. Some are gay but they choose to be straight because of social influences being so strong. That's unhealthy too to force yourself against what you want based on social influences.
I honestly don't know if homosexuality is a sin or not. God has never told me the answer, I will not read the Bible with my wisdom or Man's Wisdom, only with God's Wisdom. But God doesn't want to give me that wisdom, at least not yet, because He's never given me an answer. I also don't care, because it doesn't matter. Other's sin is none of my business. My job is to love someone, regardless if they are gay or not. If they are gay and it's not a sin, I won't be doing any harm by loving them. If they are gay and it is a sin, then it's still none of my business. Other's sin isn't my business unless God makes it my business.
Man doesn't convict people of sins. Only God convicts. If God never convicts someone of being homosexual, then that's okay, because it's none of my business. Even if it's a sin or not, God may be too busy convicting them about a billion other sins they have and never get around to homosexuality. Or maybe it's not a sin and that's why they don't get convicted. I don't care, because it's none of human's business. Other's sin is between that person and God ONLY.
The only time you should ever ask "Is homosexuality a sin?" is to yourself, and if you feel you are a homosexual. Otherwise, since it doesn't deal with YOU, it's none of your business to tell people what is or isn't sin, unless God specifically makes it your business. And honestly I don't believe God is going to tell every single Christian to speak out against homosexuality. Maybe one or two in private with a close friend, but never in mass infront of everyone so all can be shamed and embarassed.