Legally married - Must I file a joint tax return ?

No. If you are married (as of the last day of the year),you may elect to file using the status of married Jointly or married Separately.
Generally, separate tax returns may be more beneficial and save both people money, when both people have earnings and taxable income and high medical or miscellaneous itemized deductions exist.
Filing separately may allow more of the phased-out itemized deductions which are based on the taxpayer(s) Adjusted Gross Income.