It depends on how your names appear on the title:
Boyfriend AND Girlfriend, or
Boyfriend OR Girlfriend
If the title has AND between the names, then both of you are required to sign the title in order to transfer or sell the car. If OR is between the names, then only one of you needs to sign the title to transfer or sell.
Sometimes, the title does not have an AND or an OR between the names, and you will have to contact your DMV to determine which way the joint ownership is presumed to be held.
I am an attorney who is only licensed in the State of Florida. My answer is general legal advice based upon what I perceive your question to be, and should not be relied upon because every person's facts and circumstances are unique, and because specific laws vary from state to state. To completely evaluate a legal issue requires reviewing and evaluating all relevant facts, applicable laws and other information. My answer does not create an attorney-client relationship, and offered for informational purposes only.