go stand there and watch them. Then if that doesn't work, film them. Watching them will probably make them move to the bedroom at least.
Advertise and start charging for visitors to come round and watch too or listen. Charge sexless couples for excitement value. There must be loads of saddos who will pay you to come round while they are humping on the sofa. Bring your mates round to have a beer. Maybe start a blog and make it your project.
They only do it because you are bothered by it, and they probably think you are prudish or 'shy'. Learn more, watch closely, become an expert. Show them you are not afraid to be there and take pics if necessary, so fight back. Get creative.
Maybe...