The point I'm trying to make is that individual religions will likely NOT be permitted to make a definition different from what the government legislates. If gay marriage is the law of the land, and a church denies that status, then the church would be liable to charges of discrimination, accused of hate crimes, etc.
If churches were able to define marriage as they saw fit, Mormons would still have plural marriage.
Would enforcement of gay marriage within a church also apply to Muslims?
"Eminent scholars of Islam, such as Sheikh ul-Islam Imam Malik, Imam Shafi amongst others, rule that the Islam disallows homosexuality and ordains a capital punishment for a person guilty of it."
Wikipedia on Homosexuality and Islam