sydney australia high court tuesday begin hear validity government plan postal vote legalize same-sex marriage challenge risk destabilize rule center-right coalition court rule plan prime minister malcolm turnbull could find preside government fracture issue endanger razor-thin parliamentary majority one non-compulsory vote couple weeks away opponents launch legal challenge say vote need back parliament twice reject national ballot case dignity equality lesbian gay bisexual transgender intersex australians anna brown director legal advocacy human right law centre lead opposition vote tell reporters melbourne believe want marriage equality postal plebiscite big question mark around legal validity conservative lawmakers threaten resign ballot policy adhere risk turnbull parliamentary majority may able stick postal vote policy group liberal politicians threaten rebel side opposition would probably end tenure leader analysts say turnbull support same-sex marriage two-thirds australians party conservative wing threaten revolt deviate policy national ballot frustrate political impasse group backbenchers year say ready vote opposition labor party secure same-sex marriage plan abandon turnbull offer postal vote rejection legal challenge offer political solution increasingly vitriolic campaign force turnbull urge side show mutual respect turnbull plea go largely unheeded however opponents same-sex marriage last week launch contentious campaign advertisement government immediately reject inaccurate since postal vote formal election subject rule political advertisements activists fear surge malicious campaign run-up concern around sustain intense campaign make question private matter two people matter public discussion elaine pearson director new york-based human right watch support same-sex marriage oppose national vote tell reuters australia 's high court hear challenge same-sex marriage vote