Since one of the legal requirements for sexual harassment is that the conduct be unwelcome, make sure your supervisor knows that you consider their conduct to be unwelcome. Tell the person that this behavior offends you. Firmly refuse all invitations for dates or other personal interaction outside of work. Don't engage in sexual banter or flirt back in response.
Direct communication, whether verbal or in writing, is better than ignoring the behavior and hoping it will go away, so if you do not feel comfortable talking to your supervisor in person, you may wish to prepare a letter to ensure that they know exactly how you feel.
If that doesn't work, you may want to tell your supervisor, your human resources department or some other department or person within your organization who has the power to stop the harassment. This does not require you to file a lawsuit or hire an attorney, and may be sufficient to resolve the problem without further legal action. However, you should be aware that the time deadline to file a legal complaint starts running on the date of the harassment, not the date which your company resolves (or fails to resolve) your complaint, so do not miss legal filing deadlines waiting on the company to resolve the situation. For more information about filing deadlines in your state, see our page on Sexual Harassment Legal Standards.