If a cop uses his authority to obtain sex it is rape.  The same goes for someone who is impersonating a cop. The sex can be consensual and it is still rape.

There was a case in Los Angeles where a cop stopped a woman who had been drinking.  Instead of arresting her, the cop let her go and arranged to meet her after he got off work.  He met her at her apartment and they ended up having consensual sex. The woman had some other emotional problems and mentioned the incident to her therapist.  The therapist reported the incident to the police.  The cop was arrested for rape.  He was convicted and sent to prison for a sentence of 15 years to life.  This occurred in California and I realize the laws are not the same in every state.  However, I am sure there is a similar law in Maryland.  The reason the law is harsher for a public official acting under the color of authority is because of the damage such an act does to the confidence the public has in the police.

CAL. PEN. CODE § 261 : California Code - Section 261 (7)

(7) Where the act is accomplished against the victim's will by threatening to use the authority of a public official to incarcerate, arrest, or deport the victim or another, and the victim has a reasonable belief that the perpetrator is a public official. As used in this paragraph, "public official" means a person employed by a governmental agency who has the authority, as part of that position, to incarcerate, arrest, or deport another. The perpetrator does not actually have to be a public official.

Link to the complete Penal Code Section:
http://codes.lp.findlaw.com/cacode/PEN/3/1/9/1/s261