"When the scene arrived we all sat there not really saying anything. Afterwards nobody talked about it, which was kind of a relief," he added.
Grint's "Harry Potter" co-star Daniel Radcliffe appears to have fewer hangups about his parents seeing him nude.
When he stripped on stage to play the lead in "Equus" last year, he was quoted as saying that he grew up "walking around naked" in his house and nudity was never an "issue."
"I grew up always walking around naked, well not always, but it was never an issue in my house. I've seen my mum naked, seen my dad naked. I don't actively seek to find them naked, but it's never been a particularly big deal."
Similarly, actress Kate Hudson revealed that she never had a problem watching her famous parents -- mother Goldie Hawn and stepfather Kurt Russell -- in sex scenes.
"It never bothered me," she told Britain's Telegraph in May. "My brothers might feel differently about it but, for me, I felt my parents laid it out for us pretty well."
"We always had a good perspective about sex," Hudson said. "When you grow up with parents in showbiz who are loud and funny and the life of the party, you get pretty relaxed about that stuff."
But that doesn't mean she'll allow her five-year-old son Ryder to watch her own sex scenes.
Hudson left Ryder behind while filming her next movie, "The Killer Inside Me," with co-star Casey Affleck. "I normally take him on the set with me, but not for that one. 'Mummy is going to be naked on the bed, smoking and getting spanked in the next scene, honey!'" she said.