The Swinger Achievement
Take part in a debauched bedroom party with several participants.
Hookers are the way forward here, so Bloodstone seems like the best bet. When they proposition you, simply give them the Thumbs Up or even better, use the Come Back to My Place expression. Do that to 2 ... or even 3 if you must and then find a bed, whether it be a house you've purchased or an inn; it works. It only takes 3 to get this achievement, you and two others.