A West Australian state Liberal MP has been accused of linking gay marriage to incest.

In a speech opposing gay marriage earlier this week, Upper House Member Nick Goiran posed a hypothetical situation about a half brother and sister who wanted to marry and have children.

He used it to argue that even though two people may love each other, marriage is not always in the best interests of children and wider society.

Mr Goiran is standing by his comments, saying the argument for same sex marriage is illogical.

He argued once the definition of marriage is altered, it is open to other fundamental changes.

"When we're having these debates in the Parliament, we need to base them on logic, not emotion," he said.

"And so I'm just asking those promoting the case for same sex marriage to ensure that their argument is sound in logic.

"I'm saying that logically then, if it's one consenting adult with another consenting adult, then you need to apply the logic to their full argument.

"I acknowledge that the basis for the argument for those arguing for change is that it's about giving full expression to a loving relationship.

"However, my view is that it's illogical to then continue to discriminate against consenting adults who are close relatives."

Greens MP Lynn MacLaren says she was shocked by the comments.

"I found them to be quite archaic. It was a surprise to me that he would say something so out of touch," she said. "And I thought, this doesn't really have anything to do with marriage equality, marriage equality is about people who love each other and want to get married. Some people say it's a conservative movement."

Labor MP Sally Talbot says the comments are uninformed and borne out of fear.

"Nowhere in the world where we have marriage equality have we had the kind of slippery slope that Nick Goiran fears," she said.

"The fact is that when you have marriage equality it does not end up with an increase in polygamy or incest."