You are entitled to file a joint income tax return upon marriage. Although this simplifies the filing process, you will more than likely discover that your tax bill is either higher or lower than when you were single. It's higher when you file together, as more of your income is taxed in the higher tax brackets. This is commonly known as the marriage tax penalty. In 2003, a tax law that intended to reduce the marriage penalty went into effect, but this law didn't get rid of the penalty for higher bracket taxpayers.
Once married, you may not file separately in an attempt to avoid the marriage penalty. Actually, filing as married filing separately can raise your taxes. For the optimal filing status for your situation you should speak with your tax adviser.