Governments should not legislate in a way that makes any religious practices illegal. Many religions are vehemently opposed to homosexuality. To legally acknowledge homosexual relations in any way would be in direct contrast to many religious teachings.
Marriage is carried out on religious grounds by religious figures. The eligibility for marriage should, therefore, be guided by religious teachings. There are many passages in the dominant religious texts that explicitly prohibit same-sex relationships and marriage. For example, in the Bible it states that man should “not lie with mankind, as with womankind; it is an abomination” (Leviticus 18:22). Any nation founded on Judeo-Christian traditions, or is committed to promoting religious freedom, must not permit same-sex marriages as it is in direct opposition to many founding religious teachings.
Marriages were never intended to be religious rituals or practices. In ancient Rome, marriage was a civil matter and fell under the jurisdiction of imperial law.  Therefore, marriage was a legislative matter long before it was a religious matter. There are also now openly gay Judeo-Christian religious figures. Reform and Conservative Judaism endorse same-sex marriage, as does the United Church of Christ. Any biblical interpretation that asserts that homosexuality is a sin is an outdated interpretation of the Bible. As such, governments are free to allow gay marriage or civil partnerships. 
[P1] Governments that promote religious freedom cannot issue laws that go against widely-held religious beliefs. [P2] Many religions do not permit homosexual relationships. [P3] Therefore, governments should not legislate in a way that condones homosexual relationships. [P4] To grant homosexuals marital or civil partnership rights would condone homosexual relationships.
Rejecting the premises
[Rejecting P2] Many religions allow homosexual relationships.