There is no law to arrest. (but in SL law is observed in breach)
There is no offence even for prostitution, but there is an offence for Managing a place for prostitution(brothel). So , if Police have evidence of such an offence they can raid such a place and temporary take the persons for investigation, but they have to be released immediately after investigation. They cannot be arrested or remanded.
But there is an Old Ordinance, Inkeepter's Ordinance this applies to all hotels and all persons are required to give their true name,when they enter in the registry, if not done there is an offence, for which there is a fine (Rs 25!!??). So, if there is evidence of wrong name being given , Police can arrest for this offence.
Again, this is the Law and Police do not always follow the law. The relief is with the Fundamental Rights case in the Supreme Court.
This is the law, but best thing is to avoid - do not go to such places.
If two consenting adults are found in a private house, Police cannot even come and enter unless without a search warrant!(Even brothels are raided with a search warrant, for their protection!)