Ask yourself this questions: if a married person could just file as single, why would there be a filing status of Married Filing Separately? If the law didn't require a married person to file as a married person, why would anyone voluntarily sign up for a filing status that had a higher tax rate? So obviously, there is no legal way for any married person to file as single.
Why did you and your spouse not file a joint return? From the IRS:
"If, at the end of your tax year, you are married and one spouse is a U.S. citizen or a resident alien and the other is a nonresident alien, you can choose to treat the nonresident as a U.S. resident."

