Nude photography, in the hands of an expert, can be a definite turn-on -- especially when two lovely young models really get into the photography, and the photographer really gets into the models.
Mission: Submission
A woman curious about domination and submission gives her partner control of her sexuality for an evening. He orchestrates a psychological and sensual foray into sexual realms she's never experienced, including the unexpected participation of another woman. No pain besides the agony of feverish desire.
Bob Tales Beating Off Bob (now known as Lubrican)has a series of stories featuring Uncle Bob. This is a story about Uncle Bob told from the view point of his niece who has a plan to get her Uncle Bob to plant his seeds in her fertile soil.
A couple decides to seek new experiences and explore their sexuality. They end up inviting another man to participate and, little by little, the exploration becomes more extensive than they anticipated.
Josh, while taking a break from his stocks and securities activities, encountered a troubled woman in Hawaii. He wound up in the middle of a debacle, between a sibling rivalry and an Adonis, but wound up making the trade of his life. The sisters/lesbian tag comes to light very, very late in the story and is not a prominent theme.