The Made in Chelsea star posed topless with a riding crop for the cover of this month's FHM magazine.
"I've always found it fun to be a bit of an exhibitionist and get my kit off," Mackintosh said.
"I am very comfortable naked. Naturally at home I would just be wearing my favourite jumper over underwear if I was just slouching about anyway.
"I think that's probably how girls look sexiest - when you are kind of vulnerable but comfortable."
Of FHM bringing her and boyfriend Professor Green together, she added: "When you guys emailed me saying, 'Can Professor Green have your number?' I was like, 'What the f**k?! That's so random, are you being serious?'
"But after you told me he was a really nice guy I agreed. Then an hour later I get a call from a number I didn't know. It was Stephen.
"It was pretty cringe, he was like, 'Hey, it's Stephen. It's Pro Green'. And we chatted for, like, an hour and we went for dinner maybe two weeks later. The rest is history!"
"I think people are so quick to judge. They want to see people as 'like that person' and 'that type of person' and just assume that those two people couldn't possibly be together.
"It's ridiculous. I think if you click with someone it doesn't matter where you're from. It's much deeper than that."
The December issue of FHM is on sale now (Thursday, October 4) and Millie Mackintosh stars in Made in Chelsea, on E4 this month.