Well that's Great! A preference setting or an optional quest that ends in a scene that you can back out of. Like if after completing the quest they make it very clear what the reward is, and then give you the option to decline or except. For example... Thank you for finding the herb I was looking for. Let's go to my room and examine its medicinal effects. Or thanks for finding the mushroom I was looking for. I think it would only be right for me to tend to your mushroom. Or Yes thats the book I was looking for... Wow I'd really like it if we tried some of these techniques out. Followed by an option of Yea sure or No I'm good. They could even add dialogs with other characters, or a description about what the item is for/does. To drive it home that this will lead to something sexual. So that when the options come to proceed or back out they're fully informed on what's about to happen. I think either one would work if you ask me. Unless they just skip and don't read the dialog.