"I don't want my grandparents to see my boobs. That's it," the actress, 33, tells Glamour in its June issue, on newsstands May 13.
"It would be weird at Christmas," she says. "And, I mean, really, if you look at the movies I have done, getting naked would never 'elevate' the picture."
Speaking of family, Alba also talks about her relationship with husband Cash Warren, whom she married six years ago this month.
"Relationships can be tough, and I was engaged before," she says. "I've only had two boyfriends. In the first one, with Michael [Weatherly, star of NCIS], I didn't have a voice yet, and he was so much older."
"But now [with Cash] there are no more surprises. At around two and a half years, you're kind of like, 'Are we in it? Are we doing this?' And the easiest part? That would be snuggling every night."
Alba also reveals that her two daughters – Honor Marie and Haven Garner, who will turn 6 and 3, respectively, this summer – were the inspiration for her starting The Honest Company, particularly in light of her own health issues as a child.
"Come hell or high water, my kids were going to be as healthy as possible," she says. "My life was tough as a kid. I was so lonely. And I felt really disconnected from my friends because I spent so much time in the hospital with grown-ups, and felt like a weirdo. It's a strange way to spend your time as a kid. All you want to do is play."