Stretch marks are a results of rapidly gaining or losing weight. During this time the stretch marks can be avoided by rubbing on 100% pure cocoa butter 3-4 times per day. Not just applying it, but rubbing it in until there is nothing left. However, your stretch marks are already established.
You can make the stretch marks nearly disappear by doing the cocoa butter now, but it will take a solid 6-8 weeks. If there is any scarring then pure vitamin e oil will do the trick. Buy some vitamin e capsules (usually used for injesting), but crack open the capsule and use the pure vitamin e oil inside. Will work for any type of scar you might have.. (i..E previous wounds, acne marks, etc.)
your other option is laser surgery, which is actually not that expensive for the benefits. Your stomach will probably cost around $2000 cdn or so.
Also, i'm sure she will not mind the stretch marks... Most girls have them on their glutes, and it's not uncommon to develop them on thighs and stomach, even breasts. You just have to let her know how much it bothers you.
If it makes you feel any better, i'm a personal trainer and I bodybuild. I went from 158lb up to 241lb (almost 100lb) of muscle with little fat in around 2 years. There was a 3 month period where I put on 40lb very quickly from eating like a champ. I look good, but the stretch marks I have will put yours to shame i'm sure. My biceps each have well over 200 strech marks... My chest has around 20 each, very thick and long. My stomach also has some vertical marks that are very noticable. My calves, thighs, ass... Heck, even my knuckles have little stretch marks on them. And yeah girls have commented on them, but hey... Take me for who I am. I'm only human. I'm still training pretty hard, but I know as soon as i'm where I want to be I just need the good ol' cocoa butter and vitamin e oil and my body will be all cleared up. :)