A WEST Australian MP has drawn criticism after comparing gay marriage to an incestuous relationship between siblings.

In a speech to WA parliament, upper house Liberal MP Nick Goiran said he wanted to speak "in defence of the traditional definition of marriage".

The South Metropolitan member then raised the hypothetical scenario of Ethan and Stephanie, who were half brother and sister but choose to get married.

"My view is, and others may disagree, that once we start to alter the definition of marriage we give it a new status quo," Mr Goiran said.

"Marriage between a husband and a wife based on love and trust has been proven to strengthen society and statistics prove this.

"If we are going to be serious about strengthening society then we have to get real and uphold traditional marriage, not blur the lines, water it down and make a significant change to the foundational unit of society."

Gay marriage became an election issue when Prime Minister Kevin Rudd promised to introduce a same-sex marriage bill within 100 days of being re-elected.

And opposition leader Tony Abbott was put on the spot this week during an impromptu interview with pop star Katy Perry about his position on same-sex marriage.

Mr Goiran's speech - which also hypothesised about his sibling couple "playing Russian roulette" with potential birth defects for their children - drew rapid criticism from parliamentary colleagues.

"Nowhere in the world where we have marriage equality have we had the kind of slippery slope that Nick Goiran fears," Labor MP Sally Talbot told the ABC.

Originally published as Bizarre gay marriage speech from WA MP