I have stretch marks and haven't ever been pregnant (or overweight!)
It's genetic, i just have really bad genes, and dread to the think what will happen when I do become pregnant!!!
My brother got them all up his back just from growth spurts, just like he had been whiped or something!
You can rub Vit E cream into them which will make them fade (as they are basically scar tissue) but I dont think anything will ever get rid of them completely.