Is homosexuality a sin?

"Homosexuality" is a hot topic everywhere in the world now. But is it a sin according to Buddhism? Does it cause a breaking of the third precept? I have heard in a sermon that people are born gay because of breaking the third precept severely in previous births and people are born as females for also breaking the third precept (not as severely I suppose). I know that where I come from if the government accepts gay marriages the monks will be the first to protest even before the church does! Is it because it is a sin? (PS: I am totally fine with someone being gay)