His comments follow a week of bitter debate after the ACT government passed a law allowing homosexual civil unions in the territory.
Mr Nicholson says same-sex couples should be able to receive the same freedoms and rights as other people and their children shouldn't be discriminated against.
He says the law's lack of formal recognition of gay relationships amounts to cruelty.
"I think it's an act of cruelty, both to them and to the children because it seems to me that people are forgetting the children in all this.
"Incidentally, there's - as far as I'm concerned - not a shred of credible evidence that children brought up in a gay relationship have any significant disadvantage in relation to other children."