Search terms used: "19th century hallucinogens", "Victorian hallucinogens", "Victorian aphrodisiacs", "hallucinogen sexual effects"
This is for the same Victorian urban fantasy story in my earlier post (a big thank-you to all the people who replied, btw! you guys were extremely helpful!). One of the subplots involves a young woman, Miss A, who is the daughter of a murdered professor (and who isn't the most mentally stable person, even before her father's death). Over the course of the story, she grows more and more obsessed with Mr. B, the policeman who's working to solve the murder.
Towards the end of the story arc, Miss A drugs Mr. B, and has sex with him while he's under the influence. She doesn't see herself as doing anything wrong - in her mind, it's like giving him a love potion in a romance story, and she believes she can make him fall in love and marry her, which would cancel out any sin committed by having sex before marriage. Needless to say, it doesn't work - Mr. B is disgusted and horrified when he realizes what's happened, and wants nothing to do with her.
My question is, what kind of drugs, available in the late 19th century, would leave a man complete incoherent and unaware of his surroundings, but still able to perform sexually? The "able to perform sexually" part is vital, because it's a plot point that Miss A gets pregnant from this incident.
I can't be vague and just say "she slipped him something", because Mr. B is a very analytical person and goes to great lengths to find out exactly what Miss A did to him. However, I also don't want to write a how-to manual, so it doesn't need to be a perfect real-world drug. A particular class/family of drugs, or a real one with similar effects that I could base a fictional version on, would be ideal.
Getting access to rare herbs and potions wouldn't be a problem - Miss A's father dabbled in the occult, and left behind an extensive collection, so she could easily find the right mixture to slip into Mr. B's tea or wine. It's just a question of knowing what kind of mixture she should go for. I've found a number of books and articles that talk about Victorian scholars experimenting with peyote and other hallucinogenic drugs used in native peoples' rituals, but not many reliable sources that talk about the sexual side effects of such drugs.
To clarify, Mr. B isn't interested in Miss A at all before this happens, and he has a strong sense of honor and propriety - basically, he'd never touch her if he was sober. He's also engaged to another woman, who he's very much in love with, so it's plausible he could hallucinate he's having sex with his wife-to-be.
I know this is a sensitive subject, and I'm reasonably confident about my ability to depict the psychological effects on the characters involved. I just need some help with the technical side of things.