The 34-year-old singer/actress and 69-year-old producer David have raised eyebrows with their age difference since they first started dating, but insisted it doesn't bother them and got engaged last year (2018).
Some people are still struggling to come to terms with the couple, however, with one person tweeting: "Is anyone else totally disturbed by @katharinemcphee and her geriatric love? #daddyissues."
Unwilling to take the criticism, Katharine hit back by writing: "oh i LOVE that demi lovato song!". "Daddy Issues" is a song from Demi's 2017 album "Tell Me You Love Me".
oh i LOVE that demi lovato song! https://t.co/uGCWfJ5eWR— Kat McPhee (@katharinemcphee) January 17, 2019
Andy Mientus, who starred alongside Katharine in hit TV show "Smash", then weighed in by tweeting: "Imagine not appreciating a dzatty (sic)", with a GIF of "Queer Eye for the Straight Guy" star Jonathan Van Ness repeatedly saying the word "basic".
Imagine not appreciating a dzatty. pic.twitter.com/lwU7yPpRH6— Andy Mientus (@andymientus) January 17, 2019
Clearly appreciative of the support, Katharine replied: "IMAGINE A WORLD LIKE THAT."
IMAGINE A WORLD LIKE THAT— Kat McPhee (@katharinemcphee) January 17, 2019
The Twitter row comes just days after Katharine enjoyed a wedding dress fitting on Tuesday, after which she admitted she had been rather emotional trying on gowns for her big day.
Katharine and David, whose engagement was first reported in July, have yet to reveal their wedding date, but it won't be their first trip down the aisle. The bride-to-be was previously married to Nick Cokas from 2008 to 2016, while David has four ex-wives. He split from his fourth wife, former model Yolanda Hadid, the mother of runway beauties Gigi Hadid and Bella Hadid, in late 2015 after four years as husband and wife.