Actually you are not filing "married but single" as there is no such status--You are filing married filing separately
If I am filing a separate return why do I have to list my spouse’s information on my return?
Even if you file separate returns (the worst way to file) you each have to list each other's SSN's and some other information on your own tax return. The IRS can then cross check to make sure you are not "double dipping" for itemized deductions, dependents, etc.
If you are in a community property state, there is more information that will be needed.