In 35 states, cops are allowed to have sex with people in custody. This is literally, and figuratively a gross abuse of power, as those who are supposed to be upholding the law are bending it to their own will. The hypocrisy, of being immune to charges while committing the same heinous crimes an officer might put someone in jail for is unacceptable. 

In order for this to be illegal, there needs to be a law that discourages police officers from abusing their power. Having sex with anyone in custody should be illegal under all circumstances. 

The bottom line is this: when in custody, and under the authority of someone, the terms of "consent" don't apply: a police officer may say someone "consented" to have sex, when, in reality, they consent because they thought they had no choice , and if that's the case, the idea of "consent" is invalid; only outside of custody can this be taken seriously.