They can do what ever they want to do as long as they have consent and are doing it in their own house.
But definitely should not be allowed in public.
The more you stop something the more it grows.
All this started when Zia started bringing strict Islamic laws against co education, prostitution, alcohol etc...
In the 1960s and 70s there was only one red light area in Karachi where there was alcohol and prostitutes and everyone knew about it, and were reluctant to go there lest they were seen in such a place by a relative or friend, after Zia tried to closed them down, they went underground and spread all over the city, now every 4th young man in Karachi is either an alcoholic or drug addict and every 4th woman a prostitute.
Pakistan was definitely more Islamic in 60s and 70s when alcohol and prostitution was legal and regulated, than it is after the 1980s thanks to the Mullahs. Otherwise we were in the space race in the 1960s now its 2013 and look at us.