Because you don't like same-sex marriage, you wreck your own? Brilliant. Like pissing on your own feet because you object to flip flops— Dean Burnett (@garwboy) June 12, 2015
Actually, they are not wrecking their marriage. They intend to continue to live together as man and wife, and they intend to have more children.
Nick Jensen explains
"My wife and I, as a matter of conscience, refuse to recognise the government’s regulation of marriage if its definition includes the solemnisation of same sex couples."
In other words, same-sex marriage reframes the legal institution of marriage. The Jensens feel that same-sex marriage devalues all marriages, including theirs, and there is no real purpose in remaining tied to an institution that they no longer value. So this divorce plan appears to be a purely symbolic protest with no real content. (Like miming pissing on your feet.)
Surprisingly, Australian law doesn't recognize a symbolic protest as a valid reason for divorce. Divorces are only granted when marriages have broken down irretrievably. So unless the Jensens are willing to fake a separation for the purpose of the Australian divorce courts, they will be prevented from carrying out their threat.
In the meantime, the gesture has created a lot of noise. Perhaps that's as much as can be expected.
Gay law change may force us to divorce (City News, 10 June 2015)
Kate Aubusson, Christian couple vow to divorce if same-sex marriage is legalised (Sydney Morning Herald, 11 June 2015)
John Griffiths, Bad news for the Jensens’ divorce plans (City News 12 June 2015)
Kirsty McCleod, If same-sex marriage is legalised, can you divorce? (@FGDFamilyLaw, 11 June 2015)
Same Sex Couple Threaten Not To Give A Shit If Other Couple Divorces (Backburner, 11 June 2015)
The Purpose of Marriage (February 2012)
#BigWeddingWeekend (Storify August 2014)