The Formula 1 star told ITV's This Morning today that marriage and kids weren't currently on the agenda for them, saying: "All in good time."
"She's amazing with kids. I think we're both pretty good with kids but we don't have time for that at the moment," he said.
"There are so many things we want to do, but the great thing is that we support each other with it. All in good time! She's still massively young and I'm still young!"
Lewis, 29, and Nicole, 36, have been together for around seven years but their hectic schedules mean they're often in completely different parts of the world.
"With our careers people perhaps don't understand that she lives in LA, I live in Monaco and we're in and out all the time," said Lewis.
"Schedules are changing all the time. She's doing Cats now which means she's here [in the UK] for the next two months and I go away and do my training. So during the year we're always apart, but people can see there's a lot of depth there - we've been together for almost seven years."
Nicole was by Lewis' side at the weekend when he won his second Formula 1 world championship in Abu Dhabi, and looked visibly emotional and super proud as he crossed the finish line.
Lewis said this week: "Having Nicole there at the weekend was special. People could see the depth of our relationship and the sheer joy we share together."
Adding of his victory, he said: "This time just feels so much sweeter, I don't know why. I think generally I just feel much more comfortable in my life and where I am and decisions I've made… the way the year has gone and how I'm handled myself. All those things."
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