Homosexuality exists in other animals as well. In fact there are complete asexual animals, bisexual ones, and creatures with both male and female reproductive organs.

So it's nothing unnatural, its not a question of chance but a questin of time until it got to humanity.

It was only regarded sinful in the past, because of religious prohibitions, which may have made sense when humans were scarce and lived in small tribes, since homosexuals didn't reproduced, which was a survival threat.

However given that today's population is pretty huge, it makes no sense anymore to have those restrictions. And the religious prohibitions are baseless anyway.