"It's an aspect of who I am just as I like rugby and drink beer and a few other things."
Mr Robertson, the MP for Wellington Central, declared his intention to seek the leadership when David Cunliffe gave notice on Saturday that he would resign tomorrow and seek a new mandate.
Deputy leader David Parker is expected to become acting leader.
Mr Robertson and Mr Cunliffe were the main rivals in last year's leadership contest, which was forced by the resignation of David Shearer who believed he was about to lose a no-confidence motion.
One of the issues at the heart of the contest is likely to be which candidate would have the widest appeal to middle New Zealand.
In that context Mr Robertson's sexuality and Mr Cunliffe's likeability may become relevant factors.
Mr Cunliffe led the party to its third election loss in a row, polling just 24.69 per cent last weekend.
Labour has now been through four leaders while Prime Minister John Key has led National - Helen Clark, Phil Goff, David Shearer and David Cunliffe.
Mr Robertson said the party needed a new generation at the helm.
He will consider going to the leadership contest on a ticket - naming who he would like to see as his deputy.
"Clearly that would give further shape to the kind of leadership that I would bring."
He agreed Jacinda Ardern was a possibility but no decision had been made and it was ultimately a caucus decision.
"What I am clear about is I think we need a new generation of leadership. That's the generational change that we require now to meet the new challenges that the party has got.
"I've got to take some time to sit down and listen to my colleagues, along with everybody else."
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