Inevitably, that decision made headlines and raised eyebrows. But six years on, Kate says they "have definitely seen a change" in attitudes towards their relationship. To them, and their team-mates, the relationship is just "normal", but they know their situation remains an unusual one, and they are aware that they will be the first married gay couple to compete at the Olympics.
"I hope there are thousands more," says Kate. "You dye your hair blonde, you dye your hair brown, you're with a man, you're with a woman. Who cares? You're just in love with who you are in love with and that's the end of it."
Despite sharing more than 600 international appearances between them, Rio feels special. "Because of what we went through, it just felt a bit more monumental this time," says Kate, who, at 36, is two years older than her wife.
After glory at London 2012, where they both won a bronze medal, Helen's career was threatened after she suffered two ruptured discs in her back within 11 months. She was left worrying that she would never be able to play again, while they both feared the injuries might have a lasting impact on the rest of their lives.
"I had back surgery and I got back and it was fine, and then 11 months later I had to have it again," Helen says. "When that happens, you wonder: is it going to last? Is it going to happen again? There was a three or four-month period where every day I woke up and did not think I could do it."
Kate, the Great Britain captain, had to shrug off the feeling of "helplessness" and carry on without her sidekick. "It was hard playing on the team without her," she says. "We played close together and would often link up. Even now, when she is playing a bit higher up the field, we still connect instinctively because I know where she is going to be."
Helen did fight back, but not in time to make the England squad for the 2014 World Cup. That was a brutal blow for both of them, with Helen devastated not to be picked after working so hard to regain her fitness, and Kate forced into the difficult situation of having to juggle her duties as a wife and her responsibilities as captain.
The tournament itself was a disaster. England lost four of their five pool games and only avoided finishing last overall by beating Belgium in a play-off for 11th place.
But now Helen is back, and playing "the best hockey of her life" in a more advanced midfield position.
"When somebody is injured, you really see them tested to their absolute max," says Kate. "When it is a long-term injury that is debilitating and might affect the rest of their life, let alone whether they can play hockey again, mentally and physically they are really put under the microscope. So to see Helen flourish and play the best hockey that she has ever played makes her such a good role model for the rest of the team."
At Great Britain's training base at the National Sports Centre in Bisham, there is genuine belief that, under Kate and Helen's leadership, this team can reach the Olympic podium.
"We have the capability to win a gold medal with this team and I think every one of us believes that," says Kate. To do so, they will need to overcome a formidable Dutch side, who took gold in London and Beijing, and won that World Cup two years ago.
The relationship between Kate and Helen, the team's two oldest players, will be crucial. While they admit they can be a little more "direct" with each other on the pitch, they insist the frustrations of hockey never spill over into life at their Reading home.
"You are always hardest on the people you know the best," says Kate. "We probably say it a little bit more direct to each other, and we say it like we wouldn't say it to other people, but we deal with it in the moment. By the time we get home it's done."
Helen did fight back, but not in time to make the England squad for the 2014 World Cup. That was a brutal blow for both of them, with Helen devastated not to be picked after working so hard to regain her fitness, and Kate forced into the difficult situation of having to juggle her duties as a wife and her responsibilities as captain.
The tournament itself was a disaster. England lost four of their five pool games and only avoided finishing last overall by beating Belgium in a play-off for 11th place.
But now Helen is back, and playing "the best hockey of her life" in a more advanced midfield position.
"When somebody is injured, you really see them tested to their absolute max," says Kate. "When it is a long-term injury that is debilitating and might affect the rest of their life, let alone whether they can play hockey again, mentally and physically they are really put under the microscope. So to see Helen flourish and play the best hockey that she has ever played makes her such a good role model for the rest of the team."
At Great Britain's training base at the National Sports Centre in Bisham, there is genuine belief that, under Kate and Helen's leadership, this team can reach the Olympic podium.
"We have the capability to win a gold medal with this team and I think every one of us believes that," says Kate. To do so, they will need to overcome a formidable Dutch side, who took gold in London and Beijing, and won that World Cup two years ago.
The relationship between Kate and Helen, the team's two oldest players, will be crucial. While they admit they can be a little more "direct" with each other on the pitch, they insist the frustrations of hockey never spill over into life at their Reading home.
"You are always hardest on the people you know the best," says Kate. "We probably say it a little bit more direct to each other, and we say it like we wouldn't say it to other people, but we deal with it in the moment. By the time we get home it's done."
That home will soon be in Holland, where they have both signed two-year deals with leading club side Bloemendaal.
The suspicion is that the Olympics could be their final hurrah on the global stage, but they are waiting until the tournament is over before they make a decision.
Retiring from international duty would mean saying farewell to their team-mates at Bisham Abbey, who have always been supportive of their marriage - the entire squad was invited to their wedding - but never afraid to wind them up about it.
They may have to put up with the inevitable banter, but it is "never an uncomfortable joke," says Kate. "They are comfortable with it, and we are comfortable with it."
