Prime Minister John Key says while he accepts people have different beliefs he doesn't judge people on their sexuality.

Since Labour MP Louisa Wall's bill to legalise gay marriage was pulled from the ballot it has sparked lively debate.

Conservative Party leader Colin Craig drew criticism after he tweeted that gay relationships were not normal.

He later insisted people chose to be gay rather than being born that way, and said that he could choose to be gay if he wanted.

Same-sex parents were not as good role models for children as heterosexual couples, he said.

Key has indicated he will support the law change the whole way through the parliamentary process.

He had not seen anything to deter him from his initial support but would listen to the debate, he said.

"I don't see anything in terms of gay marriage that would undermine my marriage to Bronagh. At the end of the day that's a very personal thing between two people and what others choose to do I actually don't see how that affects that."

However, he accepted other people believed differently.

Key said it seemed likely people were born gay "although I can't claim any sort of medical rationale".

"What causes someone to be heterosexual in my case, or homosexual in the case of others is something I can't explain to you."

- © Fairfax NZ News