Not every religion has to have St. Augustine's attitude to sex. Why even in our culture marriages are celebrated in a church, everyone present knows what is going to happen that night, but that doesn't prevent it being a religious ceremony. ↗
No Congress ever has seen fit to amend the Constitution to address any issue related to marriage. No Constitutional Amendment was needed to ban polygamy or bigamy, nor was a Constitutional Amendment needed to set a uniform age of majority to ban child marriages. ↗