No, unmarried couples are not able to file a joint tax return. If you were married as of 12/31/15, you can file jointly, otherwise you can’t.
Note: More information on this topic is provided below.
https://www.irs.gov/uac/What-Is-My-Filing-Status%3F
Can-an-Unmarried-Couple-File-Jointly
https://ttlc.intuit.com/replies/3302320
