I honestly don't think failing in bed FOR THE FIRST FREAKING TIME would make someone you married leave you. A relationship isn't all about sex. I'd think that my partner would accept for who I am, especially since she agreed to marry me even before sex. Even if I do fail, I can learn.
As deejayy said,wait for a chance that's worth, not until you get married. And I talk that with my girl friends and they say it's no problem having no experience because you can improve your skill in less than 3 weeks :3