Comedian Alan Carr told Prince William he was "not hitting on his wife" after complimenting Kate at the Royal Variety Performance.
Greeting the couple at the show at Royal Albert Hall, Mr Carr told Kate: "You look absolutely beautiful."
Addressing the duke, the star then said: "Sir, I am sure you do not know who I am, but I am not hitting on your wife."
Mr Carr, 45, is gay and married party planner Paul Drayton, 46, in 2018.
The pair have been together for more than 10 years.
After Kate arrived, she was given a bunch of flowers by a 10-year-old girl.
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Meanwhile, a 10-year-old boy handed the show programme to Prince William, who asked him what kind of music he likes.
The Queen wrote to the Royal Variety Charity to send her best wishes for the event and celebrate 100 years of patronage from the Royal Family.
Proceeds from the show help people in the entertainment industry in need of care and assistance.
Due to the pandemic, the event last year had been held in front of a virtual audience at the Blackpool Opera House.
This year's showcase features performances from Years & Years, James Blunt, Elvis Costello, Gregory Porter and Anne-Marie.
The cast of Moulin Rouge! The Musical and the Royal Shakespeare Company's production of Matilda the Musical are also on the schedule.
Cirque du Soleil is due to perform part of their latest show Luzia, while former America's Got Talent contestants The Messoudi Brothers will deliver an acrobatic act.
There will also be comedy from Judi Love, Bill Bailey and Josh Widdicombe.