So I see two arguments being made here;

The first is if MA has a law on gender use bathrooms. The answer is yes and no. MA enacted laws in 2016 to protect gender identities and public bathrooms, making it illegal to discriminate based on a persons gender. So if you were biological male, you identify as female then you are permitted to use female restroom. This is only the case for public restrooms however, private business or commercial lease properties that are closed to the public dont abide by this.

However, MA discrimination laws prevent gender discrimination for bathrooms leading to the 2nd argument;

Can an office owner or business bar someone from using a specific bathroom? Well if that person identifies as a different gender then the one they were born with, then no, its a protected right.

All that said though, in your situation where your a (Im making assumptions based on your post) your a male who identifies as a male, your office manager or owner has the right to bar you from using a female restroom.