She probably does have stretch marks, a lot, if not most, women do. She's just great at hiding them, and in magazines there is the magic of photoshop and airbrushing. Regardless, she'd still be beautiful.
Stretch marks don't make a person ugly. If they are built nice, they are, and if they have a beautiful face, they do. I don't think stretch marks, which are largely determined by genetics, and not really something people can control, dictate someone's beauty. There are things that you can do to prevent and reduce the size of them, but a lot of times, those things just don't work.
I don't think stretch marks make someone less attractive, just like I don't think a handsome man, or beautiful girl is made less handsome or beautiful if they have a scar on their face, or something of that nature.
I developed pretty quickly in elementary and middle school, and have marks in the places that you mentioned above, and my husband couldn't care less. He doesn't even notice them, because over time they fade, and he really just doesn't mind. It's not impacted our relationship negatively, and he still thinks I'm beautiful. In the beginning I was worried about him seeing them, but then I realized, they weren't a big deal to him, and now they aren't to me.