While it is arguable as to whether society should permit gay marriage, nothing permits society to shun a group of people for any reason, be it on the basis of race, religion, ability, sex, sexual orientation, wealth, etc. Seeing as the majority of people who identify as being homosexual did not choose to have these tendencies, it would be completely unfair to discriminate against them for this reason.
Shunning people due to sexual orientation is not only cruel, but it is also hypocritical. You can't use the "but God hates fags" excuse because he didn't say that. He just said gays can't have sex. You can't use the "but gay guys might flirt with me" line, especially if you are not even cute to girls.