By Jon Herskovitz

AUSTIN, Texas (Reuters) - Two Houston taxpayers, backed by Texas Republican leaders, on Wednesday challenged Houston's spousal benefits for same-sex couples, asking the state's highest civil court to re-examine parts of a landmark 2015 U.S. Supreme Court ruling that legalized gay marriage.

Lawyers for Houston said in opening arguments at the Texas Supreme Court that the U.S. Supreme Court has decided that same-sex marriages should be treated equally nationwide and its decision means Houston is obliged to provide the benefits.

Human rights groups contend the case tries to erode the U.S. Supreme Court's decision on an already-settled matter. Petitioners countered the top U.S. court's decision on same-sex marriage is poorly reasoned and the couples are not entitled to the spousal employment benefits.

"A state court’s ultimate obligation is to the Constitution, not to the jargon and innovations created by Supreme Court justices," the petitioners said in court papers.

Jonathan Mitchell, a petitioners' lawyer, told the Texas court it should be up to the state to decide whether to extend spousal benefits to same-sex couples.

He said Texas, which has a state constitutional amendment banning same-sex marriage, has an interest in procreation and opposite-sex unions are the only relationships that can produce the offspring needed for economic growth.

Social conservatives say the U.S. Supreme Court overstepped its bounds in legalizing gay marriage and have tried to fight the ruling by saying it is an affront to constitutionally protected religious liberty.

Douglas Alexander, a lawyer for Houston, said the city is not arguing that spousal benefits are a fundamental right but all marriages should be treated equally. The city had extended the benefits to same-sex couples legally married in other states before the U.S. Supreme Court decision.

"If the state or the city - any governmental entity - affords benefits to opposite-sex couples, under the Equal Protection Clause and the Due Process clause, they must also provide them to same sex (couples)," he told the court.

After the highest civil court in Texas in September 2016 declined to take up the case from the taxpayers, the state's Republican governor and other Republican politicians filed briefs in October 2016 urging the all-Republican justices of the state Supreme Court to take up the matter.

"Not only are Texas Republicans wasting taxpayer dollars on a backwards crusade against loving couples and families, but they’re idiotically challenging a case settled by the U.S. Supreme Court and overwhelming public support," left-leaning Progress Texas said in a statement.

(Editing by Matthew Lewis)