Over time, almost all religions have eventually determined that sin must be based on free will and the choice to be something. That goes well past choosing to perform any specific behavior. It's been clear for at least 15 years that sexualities are neurological, endocrine-based (eg which pheromones someone reacts to), visual - and have a strong genetic component (no, it's not likely a single 'gay' gene will be found; the development of the pertinent brain locations in utero is far more complex than say, the development of eye color).
Two thousand years ago, epilepsy was thought to be possession by demons. In some parts of the world, mental illnesses such as bipolar and even tinnitus are *still* believed to be caused by moral failings and therefore are signs of sin. As was/is albinism (albino-ism), dwarfism and other atypical developments. Funny thing is, from all current findings, sexuality is the *normal* development of very specific brain locations - it just happens in the direction opposite that expected based on birth assigned sex & internal gender identity.
Point is, there is no choice or free will in being gay. And it's a false argument when religious leaders then try to slide the argument to "choosing to engage in gay behavior". Strongest reason is because God (Yaweh, Allah, etc) made the person that way & so to *choose* not fulfill the design that deity had for them would indeed be a sin in most religions.
And anyway, this all stems from Abrahamic religions needing to enforce that men are superior to women, but that's another discussion altogether....
_________________
“For small creatures such as we the vastness is bearable only through love.”
―Carl Sagan